Doubtless the factors that have played a part in molding the forms that have appeared in the procession of life upon our globe have been numerous, and, in addition to those that have been indicated, Osborn very aptly suggests that there may be undiscovered factors of evolution. Within a few years De Vries has brought into prominence the idea of sudden transformations leading to new species, and has accounted for organic evolution on that basis. Further consideration of this theory, however, will be postponed, while in the present chapter we shall endeavor to bring out the salient features of the theories of Lamarck and Darwin, without going into much detail regarding them.
Lamarck
Lamarck was the first to give a theory of evolution that has retained a place in the intellectual world up to the present time, and he may justly be regarded as the founder of that doctrine in the modern sense. The earlier theories were more restricted in their reach than that of Lamarck. Erasmus Darwin, his greatest predecessor in this field of thought, announced a comprehensive theory, which, while suggestive and forceful in originality, was diffuse, and is now only of historical importance. The more prominent writers on evolution in the period prior to Lamarck will be dealt with in the chapter on the Rise of Evolutionary Thought.
Lamarck was born in 1744, and led a quiet, monotonous life, almost pathetic on account of his struggles with poverty, and the lack of encouragement and proper recognition by his contemporaries. His life was rendered more bearable, however, even after he was overtaken by complete blindness, by the intellectual atmosphere that he created for himself, and by the superb confidence and affection of his devoted daughter Cornélie, who sustained him and made the truthful prediction that he would be recognized by posterity ("La postérité vous honorera").
His Family.—He came of a military family possessing some claims to distinction. The older name of the family had been de Monet, but in the branch to which Lamarck belonged the name had been changed to de Lamarque, and in the days of the first Republic was signed plain Lamarck by the subject of this sketch. Jean Baptiste Lamarck was the eleventh and last child of his parents. The other male members of the family having been provided with military occupations, Jean was selected by his father, although against the lad's own wish, for the clerical profession, and accordingly was placed in the college of the Jesuits at Amiens. He did not, however, develop a taste for theological studies, and after the death of his father in 1760 "nothing could induce the incipient abbé, then seventeen years of age, longer to wear his bands."
His ancestry asserted itself, and he forsook the college to follow the French army that was then campaigning in Germany. Mounted on a broken-down horse which he had succeeded in buying with his scanty means, he arrived on the scene of action, a veritable raw recruit, appearing before Colonel Lastic, to whom he had brought a letter of recommendation.
Military Experience.—The Colonel would have liked to be rid of him, but owing to Lamarck's persistence, assignedhim to a company; and, being mounted, Lamarck took rank as a sergeant. During his first engagement his company was exposed to the direct fire of the enemy, and the officers one after another were shot until Lamarck by order of succession was in command of the fourteen remaining grenadiers. Although the French army retreated, Lamarck refused to move with his squad until he received directions from headquarters to retire. In this his first battle he showed the courage and the independence that characterized him in later years.
Adopts Natural Science.—An injury to the glands of the neck, resulting from being lifted by the head in sport by one of his comrades, unfitted him for military life, and he went to Paris and began the study of medicine, supporting himself in the mean time by working as a bank clerk. It was in his medical course of four years' severe study that Lamarck received the exact training that was needed to convert his enthusiastic love for science into the working powers of an investigator. He became especially interested in botany, and, after a chance interview with Rousseau, he determined to follow the ruling passion of his nature and devote himself to natural science. After about nine years' work he published, in 1778, hisFlora of France, and in due course was appointed to a post in botany in the Academy of Sciences. He did not hold this position long, but left it to travel with the sons of Buffon as their instructor. This agreeable occupation extended over two years, and he then returned to Paris, and soon after was made keeper of the herbarium in the Royal Garden, a subordinate position entirely beneath his merits. Lamarck held this poorly paid position for several years, and was finally relieved by being appointed a professor in the newly establishedJardin des Plantes.
He took an active part in the reorganization of the Royal Garden (Jardin du Roi) into theJardin des Plantes. When, during the French Revolution, everything that was suggestive of royalty became obnoxious to the people, it was Lamarck who suggested in 1790 that the name of the King's Garden be changed to that of the Botanical Garden (Jardin des Plantes). The Royal Garden and the Cabinet of Natural History were combined, and in 1793 the name Jardin des Plantes proposed by Lamarck was adopted for the institution.
It was through the endorsements of Lamarck and Geoffroy Saint-Hilaire that Cuvier was brought into this great scientific institution; Cuvier, who was later to be advanced above him in the Jardin and in public favor, and who was to break friendship with Lamarck and become the opponent of his views, and who also was to engage in a memorable debate with his other supporter, Saint-Hilaire.
The portrait of Lamarck shown in Fig. 112 is one not generally known. Its date is undetermined, but since it was published in Thornton'sBritish Plantsin 1805, we know that it was painted before the publication of Lamarck'sPhilosophie Zoologique, and before the full force of the coldness and heartless neglect of the world had been experienced. In his features we read supremacy of the intellect, and the unflinching moral courage for which he was notable. Lamarck has a more hopeful expression in this portrait than in those of his later years.
Fig. 112.—Lamarck, 1774-1829.From Thornton'sBritish Plants, 1805.
Lamarck Changes from Botany to Zoölogy.—Until 1794, when he was fifty years of age, Lamarck was devoted to botany, but on being urged, after the reorganization of theJardin du Roi, to take charge of the department of invertebrates, he finally consented and changed from the study of plants to that of animals. This change had profound influence in shaping his ideas. He found the invertebrates in great confusion, and set about to bring order out of chaos, an undertaking in which, to his credit be it acknowledged,he succeeded. The fruit of his labors, the Natural History of Invertebrated Animals (Historie naturelle des Animaux sans Vertèbres, 1815-1822), became a work of great importance. He took hold of this work, it should be remembered, as an expert observer, trained to rigid analysisby his previous critical studies in botany. In the progress of the work he was impressed with the differences in animals and the difficulty of separating one species from another. He had occasion to observe the variations produced in animals through the influence of climate, temperature, moisture, elevation above the sea-level, etc.
He observed also the effects of use and disuse upon the development of organs: the exercise of an organ leading to its greater development, and the disuse to its degeneration. Numerous illustrations are cited by Lamarck which serve to make his meaning clear. The long legs of wading birds are produced and extended by stretching to keep above the water; the long neck and bill of storks are produced by their habit of life; the long neck of the giraffe is due to reaching for foliage on trees; the web-footed birds, by spreading the toes when they strike the water, have stimulated the development of a membrane between the toes, etc. In the reverse direction, the loss of the power of flight in the "wingless" bird of New Zealand is due to disuse of the wings; while the loss of sight in the mole and in blind cave animals has arisen from lack of use of eyes.
The changes produced in animal organization in this way were believed to be continued by direct inheritance and improved in succeeding generations.
He believed also in a perfecting principle, tending to improve animals—a sort of conscious endeavor on the part of the animal playing a part in its better development. Finally, he came to believe that the agencies indicated above were the factors of the evolution of life.
His Theory of Evolution.—All that Lamarck had written before he changed from botany to zoölogy (1794) indicates his belief in the fixity of species, which was the prevailing notion among naturalists of the period. Then, in 1800, we find him apparently all at once expressing a contrary opinion,and an opinion to which he held unwaveringly to the close of his life. It would be of great interest to determine when Lamarck changed his views, and upon what this radical reversal of opinion was based; but we have no sure record to depend upon. Since his theory is developed chiefly upon considerations of animal life, it is reasonable to assume that his evolutionary ideas took form in his mind after he began the serious study of animals. Doubtless, his mind having been prepared and his insight sharpened by his earlier studies, his observations in a new field supplied the data which led him directly to the conviction that species are unstable. As Packard, one of his recent biographers, points out, the first expression of his new views of which we have any record occurred in the spring of 1800, on the occasion of his opening lecture to his course on the invertebrates. This avowal of belief in the extensive alteration of species was published in 1801 as the preface to hisSystème des Animaux sans Vertèbres. Here also he foreshadowed his theory of evolution, saying that nature, having formed the simplest organisms, "then with the aid of much time and favorable circumstances ... formed all the others." It has been generally believed that Lamarck's first public expression of his views on evolution was published in 1802 in hisRecherches sur l'Organisation des Corps Vivans, but the researches of Packard and others have established the earlier date.
Lamarck continued for several years to modify and amplify the expression of his views. It is not necessary, however, to follow the molding of his ideas on evolution as expressed in the opening lectures to his course in the years 1800, 1802, 1803, and 1806, since we find them fully elaborated in hisPhilosophie Zoologique, published in 1809, and this may be accepted as the standard source for the study of his theory. In this work he states two propositionsunder the name of laws, which have been translated by Packard as follows:
"First Law: In every animal which has not exceeded the term of its development, the more frequent and sustained use of any organ gradually strengthens this organ, develops and enlarges it, and gives it a strength proportioned to the length of time of such use; while the constant lack of use of such an organ imperceptibly weakens it, causing it to become reduced, progressively diminishes its faculties, and ends in its disappearance.
"Second Law: Everything which nature has caused individuals to acquire or lose by the influence of the circumstances to which their race may be for a long time exposed, and consequently by the influence of the predominant use of such an organ, or by that of the constant lack of use of such part, it preserves by heredity and passes on to the new individuals which descend from it, provided that the changes thus acquired are common to both sexes, or to those which have given origin to these new individuals.
"These are the two fundamental truths which can be misunderstood only by those who have never observed or followed nature in its operations," etc. The first law embodies the principle of use and disuse, the second law that of heredity.
In 1815 his theory received some extensions of minor importance. The only points to which attention need be called are that he gives four laws instead of two, and that a new feature occurs in the second law in the statement that the production of a new organ is the result of a new need (besoin) which continues to make itself felt.
Simplified Statement of Lamarck's Views.—For practical exposition the theory maybe simplified into two sets of facts: First, those to be classed under variation; and, second, those under heredity. Variations of organs, according to Lamarck,arise in animals mainly through use and disuse, and new organs have their origin in a physiological need. A new need felt by the animal expresses itself on the organism, stimulating growth and adaptations in a particular direction. This part of Lamarck's theory has been subjected to much ridicule. The sense in which he employs the wordbesoinhas been much misunderstood; when, however, we take into account that he uses it, not merely as expressing a wish or desire on the part of the animal, but as the reflex action arising from new conditions, his statement loses its alleged grotesqueness and seems to be founded on sound physiology.
Inheritance.—Lamarck's view of heredity was uncritical; according to his conception, inheritance was a simple, direct transmission of those superficial changes that arise in organs within the lifetime of an individual owing to use and disuse. It is on this question of the direct inheritance of variations acquired in the lifetime of an individual that his theory has been the most assailed. The belief in the inheritance of acquired characteristics has been so undermined by experimental evidence that at the present time we can not point to a single unchallenged instance of such inheritance. But, while Lamarck's theory has shown weakness on that side, his ideas regarding the production of variations have been revived and extended.
Variation.—The more commendable part of his theory is the attempt to account for variation. Darwin assumed variation, but Lamarck attempted to account for it, and in this feature many discerning students maintain that the theory of Lamarck is more philosophical in its foundation than that of Darwin.
In any theory of evolution we must deal with the variation of organisms and heredity, and thus we observe that the two factors discussed by Lamarck are basal. Although it must be admitted that even to-day we know little about eithervariation or heredity, they remain basal factors in any theory of evolution.
Time and Favorable Conditions.—Lamarck supposed a very long time was necessary to bring about the changes which have taken place in animals. The central thought of time and favorable conditions occurs again and again in his writings. The following quotation is interesting as coming from the first announcement of his views in 1800:
"It appears, as I have already said, thattimeandfavorable conditionsare the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she has it always at her disposal.
"As to the circumstances of which she has had need and of which she makes use every day in order to cause her productions to vary, we can say that in a manner they are inexhaustible.
"The essential ones arising from the influence and from all the environing media, from the diversity of local causes, of habits, of movements, of action, finally of means of living, of preserving their lives, of defending themselves, of multiplying themselves, etc. Moreover, as the result of these different influences, the faculties, developed and strengthened by use, become diversified by the new habits maintained for long ages, and by slow degrees the structure, the consistence—in a word, the nature, the condition of the parts and of the organs consequently participating in all these influences, became preserved and were propagated by heredity (génération)." (Packard's translation.)
Salient Points.—The salient points in Lamarck's theory may be compacted into a single sentence: It is a theory of the evolution of animal life, depending upon variations brought about mainly through use and disuse of parts, and also by responses to external stimuli, and the directinheritance of the same. His theory is comprehensive, so much so that he includes mankind in his general conclusions.
Lamarck supposed that an animal having become adapted to its surroundings would remain relatively stable as to its structure. To the objection raised by Cuvier that animals from Egypt had not changed since the days when they were preserved as mummies, he replied that the climate of Egypt had remained constant for centuries, and therefore no change in its fauna was to be expected.
Species.—Since the question of the fixity of species is the central one in theories of evolution, it will be worth while to quote Lamarck's definition of species: "All those who have had much to do with the study of natural history know that naturalists at the present day are extremely embarrassed in defining what they mean by the word species.... We callspeciesevery collection of individuals which are alike or almost so, and we remark that the regeneration of these individuals conserves the species and propagates it in continuing successively to reproduce similar individuals." He then goes on with a long discussion to show that large collections of animals exhibit a great variation in species, and that they have no absolute stability, but "enjoy only a relative stability."
Herbert Spencer adopted and elaborated the theory of Lamarck. He freed it from some of its chief crudities, such as the idea of an innate tendency toward perfection. In many controversies Mr. Spencer defended the idea of the transmission of acquired characters. The ideas of Lamarck have, therefore, been transmitted to us largely in the Spencerian mold and in the characteristic language of that great philosopher. There has been but little tendency to go to Lamarck's original writings. Packard, whose biography of Lamarck appeared in 1901, has made a thorough analysisof his, writings and had incidentally corrected several erroneous conception.
Neo-Lamarckism.—The ideas of Lamarck regarding the beginning of variations have been revived and accorded much respect under the designation of Neo-Lamarckism. The revival of Lamarckism is especially owing to the palæontological investigations of Cope and Hyatt. The work of E.D. Cope in particular led him to attach importance to the effect of mechanical and other external causes in producing variation, and he points out many instances of use-inheritance. Neo-Lamarckism has a considerable following; it is a revival of the fundamental ideas of Lamarck.
Darwin's Theory
While Lamarck's theory rests upon two sets of facts, Darwin's is founded on three:viz., the facts of variation, of inheritance, and of natural selection. The central feature of his theory is the idea of natural selection. No one else save Wallace had seized upon this feature when Darwin made it the center of his system. On account of the part taken by Wallace simultaneously with Darwin in announcing natural selection as the chief factor of evolution, it is appropriate to designate this contribution as the Darwin-Wallace principle of natural selection. The interesting connection between the original conclusions of Darwin and Wallace is set forth in Chapter XIX.
Variation.—It will be noticed that two of the causes assigned by Darwin are the same as those designated by Lamarck, but their treatment is quite different. Darwin (Fig. 113) assumed variation among animals and plants without attempting to account for it, while Lamarck undertook to state the particular influences which produce variation, and although we must admit that Lamarck was not entirely successful in this attempt, the fact that he undertook the task places his contribution at the outset on a very high plane.
Fig. 113.—Charles Darwin, 1809-1882.
The existence of variation as established by observation is unquestioned. No two living organisms are exactly alike at the time of their birth, and even if they are brought up together under identical surroundings they vary. The variation of plants and animals under domestication is so conspicuous and well known that this kind of variation was the first to attract attention. It was asserted that these variations were perpetuated because the forms had been protected by man, and it was doubted that animals varied to any considerable extent in a state of nature. Extended collections and observations in field and forest have, however, set this question at rest.
If crows or robins or other birds are collected on an extensive scale, the variability of the same species will be evident. Many examples show that the so-called species differ greatly in widely separated geographical areas, but collections from the intermediate territory demonstrate that the variations are connected by a series of fine gradations. If, for illustration, one should pass across the United States from the Atlantic to the Pacific coast, collecting one species of bird, the entire collection would exhibit wide variations, but the extremes would be connected by intermediate forms.
The amount of variation in a state of nature is much greater than was at first supposed, because extensive collections were lacking, but the existence of wide variation is now established on the basis of observation. This fact of variation among animals and plants in the state of nature is unchallenged, and affords a good point to start from in considering Darwinism.
Inheritance.—The idea that these variations are inherited is the second point. But what particular variations will be preserved and fostered by inheritance, and on whatprinciple they will be selected, is another question—and a notable one. Darwin's reply was that those variations which are of advantage to the individual will be the particular ones selected by nature for inheritance. While Darwin implies the inheritance of acquired characteristics, his theory of heredity was widely different from that of Lamarck. Darwin's theory of heredity, designated the provisional theory of pangenesis, has been already considered (see Chapter XIV).
Natural Selection.—Since natural selection is the main feature of Darwin's doctrine, we must devote more time to it. Darwin frequently complained that very few of his critics took the trouble to find out what he meant by the term natural selection. A few illustrations will make his meaning clear. Let us first think of artificial selection as it is applied by breeders of cattle, fanciers of pigeons and of other fowls, etc. It is well known that by selecting particular variations in animals and plants, even when the variations are slight, the breeder or the horticulturalist will be able in a short time to produce new races of organic forms. This artificial selection on the part of man has given rise to the various breeds of dogs, the 150 different kinds of pigeons, etc., all of which breed true. The critical question is, Have these all an individual ancestral form in nature? Observation shows that many different kinds—as pigeons—may be traced back to a single ancestral form, and thus the doctrine of the fixity of species is overthrown.
Now, since it is demonstrated by observation that variations occur, if there be a selective principle at work in nature, effects similar to those caused by artificial selection will be produced. The selection by nature of the forms fittest to survive is what Darwin meant by natural selection. We can never understand the application, however, unless we take into account the fact that while animals tend to multiply in geometrical progression, as a matter of fact thenumber of any one kind remains practically constant. Although the face of nature seems undisturbed, there is nevertheless a struggle for existence among all animals.
This is easily illustrated when we take into account the breeding of fishes. The trout, for illustration, lays from 60,000 to 100,000 eggs. If the majority of these arrived at maturity and gave rise to progeny, the next generation would represent a prodigious number, and the numbers in the succeeding generations would increase so rapidly that soon there would not be room in the fresh waters of the earth to contain their descendants. What becomes of the immense number of fishes that die? They fall a prey to others, or they are not able to get food in competition with other more hardy relatives, so that it is not a matter of chance that determines which ones shall survive; those which are the strongest, the better fitted to their surroundings, are the ones which will be perpetuated.
The recognition of this struggle for existence in nature, and the consequent survival of the fittest, shows us more clearly what is meant by natural selection. Instead of man making the selection of those particular forms that are to survive, it is accomplished in the course of nature. This is natural selection.
Various Aspects of Natural Selection.—Further illustrations are needed to give some idea of the various phases of natural selection. Speed in such animals as antelopes may be the particular thing which leads to their protection. It stands to reason that those with the greatest speed would escape more readily from their enemies, and would be the particular ones to survive, while the weaker and slower ones would fall victims to their prey. In all kinds of strain due to scarcity of food, inclemency of weather, and other untoward circumstances, the forms which are the strongest, physiologically speaking, will have the best chance to weather thestrain and to survive. As another illustration, Darwin pointed out that natural selection had produced a long-legged race of prairie wolves, while the timber wolves, which have less occasion for running, are short-legged.
We can also see the operation of natural selection in the production of the sharp eyes of birds of prey. Let us consider the way in which the eyes of the hawk have been perfected by evolution. Natural selection compels the eye to come up to a certain standard. Those hawks that are born with weak or defective vision cannot cope with the conditions under which they get their food. The sharp-eyed forms would be the first to discern their prey, and the most sure in seizing upon it. Therefore, those with defective vision or with vision that falls below the standard will be at a very great disadvantage. The sharp-eyed forms will be preserved by a selective process. Nature selects, we may say, the keener-eyed birds of prey for survival, and it is easy to see that this process of natural selection would establish and maintain a standard of vision.
But natural selection tends merely to adapt animals to their surroundings, and does not always operate in the direction of increasing the efficiency of the organ. We take another illustration to show how Darwin explains the origin of races of short-winged beetles on certain oceanic islands. Madeira and other islands, as Kerguelen island of the Indian Ocean, are among the most windy places in the world. The strong-winged beetles, being accustomed to disport themselves in the air, would be carried out to sea by the sudden and violent gales which sweep over those islands, while the weaker-winged forms would be left to perpetuate their kind. Thus, generation after generation, the strong-winged beetles would be eliminated by a process of natural selection, and there would be left a race of short-winged beetles derived from long-winged ancestors. In this case the organs arereduced in their development, rather than increased; but manifestly the short-winged race of beetles is better adapted to live under the particular conditions that surround their life in these islands.
While this is not a case of increase in the particular organ, it illustrates a progressive series of steps whereby the organism becomes better adapted to its surroundings. A similar instance is found in the suppression of certain sets of organs in internal parasites. For illustration, the tapeworm loses particular organs of digestion for which it does not have continued use; but the reproductive organs, upon which the continuance of its life depends, are greatly increased. Such cases as the formation of short-winged beetles show us that the action of natural selection is not always to preserve what we should call the best, but simply to preserve the fittest. Development, therefore, under the guidance of natural selection is not always progressive. Selection by nature does not mean the formation and preservation of the ideally perfect, but merely the survival of those best fitted to their environment.
Color.—The various ways in which natural selection acts are exceedingly diversified. The colors of animals may be a factor in their preservation, as the stripes on the zebra tending to make it inconspicuous in its surroundings. The stripes upon the sides of tigers simulate the shadows cast by the jungle grass in which the animals live, and serves to conceal them from their prey as well as from enemies. Those animals that assume a white color in winter become thereby less conspicuous, and they are protected by their coloration.
As further illustrating color as a factor in the preservation of animals, we may cite a story originally told by Professor E.S. Morse. When he was collecting shells on the white sand of the Japanese coast, he noticed numerous white tiger-beetles, which could scarcely be seen against the whitebackground. They could be detected chiefly by their shadows when the sun was shining. As he walked along the coast he came to a wide band of lava which had flowed from a crater across the intervening country and plunged into the sea, leaving a broad dark band some miles in breadth across the white sandy beach. As he passed from the white sand to the dark lava, his attention was attracted to a tiger-beetle almost identical with the white one except as to color. Instead of being white, it was black. He found this broad, black band of lava inhabited by the black tiger beetle, and found very few, if any, of the white kind. This is a striking illustration of what has occurred in nature. These two beetles are of the same species, and in examining the conditions under which they grow, it is discovered that out of the eggs laid by the original white forms, there now and then appears one of a dusky or black color. Consider how conspicuous this dark object would be against the white background of sand. It would be an easy mark for the birds of prey that fly about, and therefore on the white surface the black beetles would be destroyed, while the white ones would be left. But on the black background of lava the conditions are reversed. There the white forms would be the conspicuous ones; as they wandered upon the black surface, they would be picked up by birds of prey and the black ones would be left. Thus we see another instance of the operation of natural selection.
Mimicry.—We have, likewise, in nature a great number of cases that are designated mimicry. For illustration, certain caterpillars assume a stiff position, resembling a twig from a branch. We have also leaf-like butterflies. The Kallima of India is a conspicuous illustration of a butterfly having the upper surface of its wings bright-colored, and the lower surface dull. When it settles upon a twig the wings are closed and the under-sides have a mark across themresembling the mid-rib of a leaf, so that the whole butterfly in the resting position becomes inconspicuous, being protected by mimicry.
One can readily see how natural selection would be evoked in order to explain this condition of affairs. Those forms that varied in the direction of looking like a leaf would be the most perfectly protected, and this feature being fostered by natural selection, would, in the course of time, produce a race of butterflies the resemblance of whose folded wings to a leaf would serve as a protection from enemies.
It may not be out of place to remind the reader that the illustrations cited are introduced merely to elucidate Darwin's theory and the writer is not committed to accepting them as explanations of the phenomena involved. He is not unmindful of the force of the criticisms against the adequacy of natural selection to explain the evolution of all kinds of organic structures.
Many other instances of the action of color might be added, such as the wearing of warning colors, those colors which belong to butterflies, grubs, and other animals that have a noxious taste. These warning colors have taught birds to leave alone the forms possessing those colors. Sometimes forms which do not possess a disagreeable taste secure protection by mimicking the colors of the noxious varieties.
Sexual Selection.—There is an entirely different set of cases which at first sight would seem difficult to explain on the principle of selection. How, for instance, could we explain the feathers in the tails of the birds of paradise, or that peculiar arrangement of feathers in the tail of the lyre-bird, or the gorgeous display of tail-feathers of the male peacock? Here Mr. Darwin seized upon a selective principle arising from the influence of mating. The male birds in becoming suitors for a particular female have been accustomed to display their tail-feathers; the one with the most attractive display excites the pairing instinct in the highest degree, and becomes the selected suitor. In this way, through the operation of a form of selection which Darwin designates sexual selection, possibly such curious adaptations as the peacock's tail may be accounted for.
It should be pointed out that this part of the theory is almost wholly discredited by biologists. Experimental evidence is against it. Nevertheless in a descriptive account of Darwin's theory it may be allowed to stand without critical comment.
Inadequacy of Natural Selection.—In nature, under the struggle for existence, the fittest will be preserved; and natural selection will operate toward the elaboration or the suppression of certain organs or certain characteristics when the elaboration or the suppression is of advantage to the animal form. Much has been said of late as to the inadequacy of natural selection. Herbert Spencer and Huxley, both accepting natural selection as one of the factors, doubted its complete adequacy.
One point is often overlooked, and should be brought out with clearness;viz., that Darwin himself was the first to point out clearly the inadequacy of natural selection as a universal law for the production of the great variety of animals and plants. In the second edition of theOrigin of Specieshe says: "But, as my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work and subsequently I placed in a most conspicuous position,—namely, at the close of the introduction—the following words: 'I am convinced that natural selection has been the main, but not the exclusive means of modification.' This has been of no avail. Great is the power of steady misrepresentation. But the history of science shows that fortunately this power does not long endure."
The reaction against the all-sufficiency of natural selection, therefore, is something which was anticipated by Darwin, and the quotation made above will be a novelty to many of our readers who supposed that they understood Darwin's position.
Confusion between Lamarck's and Darwin's Theories.—Besides the failure to understand what Darwin has written, there is great confusion, both in pictures and in writings, in reference to the theories of Darwin and Lamarck. Poulton illustrated a state of confusion in one of his lectures on the theory of organic evolution, and the following instances are quoted from memory.
We are most of us familiar with such pictures as the following: A man standing and waving his arms; in the next picture these arms and hands become enlarged, and in the successive pictures they undergo transformations into wings, and the transference is made into a flying animal.
Such pictures are designated "The origin of flight after Darwin." The interesting circumstance is this, that the illustration does not apply to Darwin's idea of natural selection at all, but is pure Lamarckism. Lamarck contended for the production of new organs through the influence of use and disuse, and this particular illustration refers to that, and not to natural selection at all.
Among the examples of ridicule to which Darwin's ideas have been exposed, we cite one verse from the song of Lord Neaves. His lordship wrote a song with a large number of verses hitting off in jocular vein many of the claims and foibles of his time. In attempting to make fun of Darwin's idea he misses completely the idea of natural selection, but hits upon the principle enunciated by Lamarck, instead. He says:
"A deer with a neck which was longer by halfThan the rest of his family's—try not to laugh—By stretching and stretching became a giraffe,Which nobody can deny."
The clever young woman, Miss Kendall, however, in herSong of the Ichthyosaurus, showed clearness in grasping Darwin's idea when she wrote:
"Ere man was developed, our brother,We swam, we ducked, and we dived,And we dined, as a rule, on each other.What matter? The toughest survived."
This hits the idea of natural selection. The other two illustrations miss it, but strike the principle which was enunciated by Lamarck. This confusion between Lamarckism and Darwinism is very wide-spread.
Darwin's book on theOrigin of Species, published in 1859, was epoch-making. If a group of scholars were asked to designate the greatest book of the nineteenth century—that is, the book which created the greatest intellectual stir—it is likely that a large proportion of them would reply that it is Darwin'sOrigin of Species. Its influence was so great in the different domains of thought that we may observe a natural cleavage between the thought in reference to nature between 1859 and all preceding time. His other less widely known books onAnimals and Plants Under Domestication, theDescent of Man, etc., etc., are also important contributions to the discussion of his theory. A brief account of Darwin, the man, will be found in Chapter XIX.
CHAPTER XVIII
THEORIES OF EVOLUTION CONTINUED: WEISMANN, DE VRIES
Weismann'sviews have passed through various stages of remodeling since his first public championship of the Theory of Descent on assuming, in 1867, the position of professor of zoölogy in the University of Freiburg. Some time after that date he originated his now famous theory of heredity, which has been retouched, from time to time, as the result of aggressive criticism from others, and the expansion of his own mental horizon. As he said in 1904, regarding his lectures on evolution which have been delivered almost regularly every year since 1880, they "were gradually modified in accordance with the state of my knowledge at the time, so that they have been, I may say, a mirror of my own intellectual evolution."
Passing over his book,The Germ Plasm, published in English in 1893, we may fairly take his last book,The Evolution Theory, 1904, as the best exposition of his conclusions. The theoretical views of Weismann have been the field of so much strenuous controversy that it will be well perhaps to take note of the spirit in which they have been presented. In the preface of his book just mentioned, he says: "I make this attempt to sum up and present as a harmonious whole the theories which for forty years I have been gradually building up on the basis of the legacy of the great workers of the past, and on the results of my own investigations and those of my fellow-workers, not because I regard the picture as incomplete or incapable of improvement, but because I believe its essential features to be correct, and because an eye-trouble which has hindered my work for many years makes it uncertain whether I shall have much more time and strength granted to me for its further elaboration."
The germ-plasm theory is primarily a theory of heredity, and only when connected with other considerations does it become the full-fledged theory of evolution known as Weismannism. The theory as a whole involves so many intricate details that it is difficult to make a clear statement of it for general readers. If in considering the theories of Lamarck and Darwin it was found advantageous to confine attention to salient points and to omit details, it is all the more essential to do so in the discussion of Weismann's theory.
In his prefatory note to the English edition ofThe Evolution TheoryThomson, the translator, summarizes Weismann's especial contributions as: "(1) the illumination of the evolution process with a wealth of fresh illustrations; (2) the vindication of the 'germ-plasm' concept as a valuable working hypothesis; (3) the final abandonment of any assumption of transmissible acquired characters; (4) a further analysis of the nature and origin of variations; and (5), above all, an extension of the selection principle of Darwin and Wallace, which finds its logical outcome in the suggestive theory of germinal selection."
Continuity of the Germ-Plasm.—Weismann's theory is designated that of continuity of the germ-plasm, and in considering it we must first give attention to his conception of the germ-plasm. As is well known, animals and plants spring from germinal elements of microscopic size; these are, in plants, the spores, the seeds, and their fertilizing agents; and, in animals, the eggs and the sperms. Now, since allanimals, even the highest developed, begin in a fertilized egg, that structure, minute as it is, must contain all hereditary qualities, since it is the only material substance that passes from one generation to another. This hereditary substance is the germ-plasm. It is the living, vital substance of organisms that takes part in the development of new generations.
Naturalists are agreed on this point, that the more complex animals and plants have been derived from the simpler ones; and, this being accepted, the attention should be fixed on the nature of the connection between generations during their long line of descent. In the reproduction of single-celled organisms, the substance of the entire body is divided during the transmission of life, and the problem both of heredity and origin is relatively simple. It is clear that in these single-celled creatures there is unbroken continuity of body-substance from generation to generation. But in the higher animals only a minute portion of the organism is passed along.
Weismann points out that the many-celled body was gradually produced by evolution; and that in the transmission of life by the higher animals the continuity is not between body-cells and their like, but only between germinal elements around which in due course new body-cells are developed. Thus he regards the body-cells as constituting a sort of vehicle within which the germ-cells are carried. The germinal elements represent the primordial substance around which the body has been developed, and since in all the long process of evolution the germinal elements have been the only form of connection between different generations, they have an unbroken continuity.
This conception of the continuity of the germ-plasm is the foundation of Weismann's doctrine. As indicated before, the general way in which he accounts for heredity is that the offspring is like the parent because it is composed of some ofthe same stuff. The rise of the idea of germinal continuity has been indicated in Chapter XIV, where it was pointed out that Weismann was not the originator of the idea, but he is nevertheless the one who has developed it the most extensively.
Complexity of the Germ-Plasm.—The germ-plasm has been molded for so many centuries by external circumstances that it has acquired an organization of great complexity. This appears from the following considerations: Protoplasm is impressionable; in fact, its most characteristic feature is that it responds to stimulation and modifies itself accordingly. These subtle changes occurring within the protoplasm affect its organization, and in the long run it is the summation of experiences that determines what the protoplasm shall be and how it will behave in development. Two masses of protoplasm differ in capabilities and potentialities according to the experiences through which they have passed, and no two will be absolutely identical. All the time the body was being evolved the protoplasm of the germinal elements was being molded and changed, and these elements therefore possess an inherited organization of great complexity.
When the body is built anew from the germinal elements, the derived qualities come into play, and the whole process is a succession of responses to stimulation. This is in a sense, on the part of the protoplasm, a repeating of its historical experience. In building the organism it does not go over the ground for the first time, but repeats the activities which it took centuries to acquire.
The evident complexity of the germ-plasm made it necessary for Weismann, in attempting to explain inheritance in detail, to assume the existence of distinct vital units within the protoplasm of the germinal elements. He has invented names for these particular units as biophors, the elementary vital units, and their combination into determinants, thelatter being united into ids, idants, etc. The way in which he assumes the interactions of these units gives to his theory a highly speculative character. The conception of the complex organization of the germ-plasm which Weismann reached on theoretical grounds is now being established on the basis of observation (see Chapter XIV, p. 313).
The Origin of Variations.—The way in which Weismann accounts for the origin of variation among higher animals is both ingenious and interesting. In all higher organisms the sexes are separate, and the reproduction of their kind is a sexual process. The germinal elements involved are seeds and pollen, eggs and sperms. In animals the egg bears all the hereditary qualities from the maternal side, and the sperm those from the paternal side. The intimate mixture of these in fertilization gives great possibilities of variations arising from the different combinations and permutations of the vital units within the germ-plasm.
This union of two germ-plasms Weismann calls amphimixis, and for a long time he maintained that the purpose of sexual reproduction in nature is to give origin to variations. Later he extended his idea to include a selection, mainly on the basis of nutrition, among the vital elements composing the germ-plasm. This is germinal selection, which aids in the production of variations.
InThe Evolution Theory, volume II, page 196, he says: "Now that I understand these processes more clearly, my opinion is that the roots of all heritable variation lie in the germ-plasm; and, furthermore, that the determinants are continually oscillating hither and thither in response to very minute nutritive changes and are readily compelled tovariation in a definite direction, which may ultimately lead to considerable variations in the structure of the species, if they are favored by personal selection, or at least if they are not suppressed by it as prejudicial."
But while sexual reproduction may be evoked to explain the origin of variation in higher animals, Weismann thought it was not applicable to the lower ones, and he found himself driven to assume that variation in single-celled organisms is owing to the direct influence of environment upon them, and thus he had an awkward assumption of variations arising in a different manner in the higher and in the simplest organisms. If I correctly understand his present position, the conception of variation as due to the direct influence of environment is being surrendered in favor of the action of germinal selection among the simplest organisms.
Extension of the Principle of Natural Selection.—These variations, once started, will be fostered by natural selection provided they are of advantage to the organism in its struggle for existence. It should be pointed out that Weismann is a consistent Darwinian; he not only adopts the principle of natural selection, but he extends the field of its operation from externals to the internal parts of the germinal elements.
"Roux and others have elaborated the idea of a struggle of the parts within the organism, and of a corresponding intra-selection; ... but Weismann, after his manner, has carried the selection-idea a step farther, and has pictured the struggle among the determining elements of the germ-cell's organization. It is at least conceivable that the stronger 'determinants,'i.e., the particles embodying the rudiments of certain qualities, will make more of the food-supply than those which are weaker, and that a selective process will ensue" (Thomson). This is the conception of germinal selection.
He has also extended the application of the general doctrine of natural selection by supplying a great number of new illustrations.
The whole theory of Weismann is so well constructed that it is very alluring. Each successive position is workedout with such detail and apt illustration that if one follows him step by step without dissent on some fundamental principle, his conclusion seems justified. As a system it has been elaborated until it makes a coherent appeal to the intellect.
Inheritance of Acquired Characters.—Another fundamental point in Weismann's theory is the denial that acquired characters are transmitted from parent to offspring. Probably the best single discussion of this subject is contained in his book onThe Evolution Theory, 1904, to which readers are referred.
A few illustrations will be in place. Acquired characters are any acquisitions made by the body-cells during the lifetime of an individual. They may be obvious, as skill in piano-playing, bicycle-riding, etc.; or they may be very recondite, as turns of the intellect, acquired beliefs, etc. Acquired bodily characters may be forcibly impressed upon the organism, as the facial mutilations practiced by certain savage tribes, the docking of the tails of horses, of dogs, etc. The question is, Are any acquired characters, physical or mental, transmitted by inheritance?
Manifestly, it will be difficult to determine on a scientific basis whether or not such qualities are inheritable. One would naturally think first of applying the test of experiment to supposed cases of such inheritances, and this is the best ground to proceed on.
It has been maintained on the basis of the classical experiments of Brown-Séquard on guinea-pigs that induced epilepsy is transmitted to offspring; and, also, on the basis of general observations, that certain bodily mutilations are inherited. Weismann's analysis of the whole situation is very incisive. He experimented by cutting off the tails of both parents of breeding mice. The experiments were carried through twenty-two generations, both parents beingdeprived of their tails, without yielding any evidence that the mutilations were inheritable.
To take one other case that is less superficial, it is generally believed that the thirst for alcoholic liquors has been transmitted to the children of drunkards, and while Weismann admits the possibility of this, he maintains that it is owing to the germinal elements being exposed to the influence of the alcohol circulating in the blood of the parent or parents; and if this be the case it would not be the inheritance of an acquired character, but the response of the organism to a drug producing directly a variation in the germ-plasm.
Notwithstanding the well-defined opposition of Weismann, the inheritance of acquired characters is still a mooted question. Herbert Spencer argued in favor of it, and during his lifetime had many a pointed controversy with Weismann. Eimer stands unalterably against Weismann's position, and the Neo-Lamarckians stand for the direct inheritance of useful variations in bodily structure. The question is still undetermined and is open to experimental observation. In its present state there are competent observers maintaining both sides, but it must be confessed that there is not a single case in which the supposed inheritance of an acquired character has stood the test of critical examination.
The basis of Weismann's argument is not difficult to understand. Acquired characters affect the body-cells, and according to his view the latter are simply a vehicle for the germinal elements, which are the only things concerned in the transmission of hereditary qualities. Inheritance, therefore, must come through alterations in the germ-plasm, and not directly through changes in the body-cells.
Fig. 114.—August Weismann, Born 1834.Permission of Charles Scribner's Sons.
Weismann, the Man.—The man who for more than forty years has been elaborating this theory (Fig. 114) is still living and actively at work in the University of Freiburg. August Weismann was born at Frankfort-on-the-Main in 1834. Hewas graduated at Göttingen in 1856, and for a short time thereafter engaged in the practice of medicine. This line of activity did not, however, satisfy his nature, and he turned to the pursuit of microscopic investigations in embryology and morphology, being encouraged in this work by Leuckart, whose name we have already met in this history. In 1863 he settled in Freiburg asprivat-docent, and has remained connected with the university ever since. From 1867 onwardhe has occupied the chair of zoölogy in that institution. He has made his department famous, especially by his lectures on the theory of descent.
He is a forceful and interesting lecturer. One of his hearers in 1896 wrote: "His lecture-room is always full, and his popularity among his students fully equals his fame among scientists."
It is quite generally known that Weismann since he reached the age of thirty has been afflicted with an eye-trouble, but the inference sometimes made by those unacquainted with his work as an investigator, that he has been obliged to forego practical work in the field in which he has speculated, is wrong. At intervals his eyes have strengthened so that he has been able to apply himself to microscopic observations, and he has a distinguished record as an observer. In embryology his studies on the development of the diptera, and of the eggs of daphnid crustacea, are well known, as are also his observations on variations in butterflies and other arthropods.
He is an accomplished musician, and during the period of his enforced inactivity in scientific work he found much solace in playing "a good deal of music." "His continuous eye trouble must have been a terrible obstacle, but may have been the prime cause of turning him to the theories with which his name is connected."
In a short autobiography published inThe Lampin 1903, although written several years earlier, he gives a glimpse of his family life. "During the ten years (1864-1874) of my enforced inactivity and rest occurred my marriage with Fräulein Marie Gruber, who became the mother of my children and was my true companion for twenty years, until her death. Of her now I think only with love and gratitude. She was the one who, more than any one else, helped me through the gloom of this period. She read much to meat this time, for she read aloud excellently, and she not only took an interest in my theoretical and experimental work, but she also gave practical assistance in it."
In 1893 he publishedThe Germ-Plasm, A Theory of Heredity, a treatise which elicited much discussion. From that time on he has been actively engaged in replying to his critics and in perfecting his system of thought.
The Mutation-Theory of De Vries.—Hugo de Vries (Fig. 115), director of the Botanical Garden in Amsterdam, has experimented widely with the growth of plants, especially the evening primrose, and has shown that different species appear to rise suddenly. The sudden variations that breed true, and thus give rise to new forms, he calls mutations, and this indicates the source of the name applied to his theory.
In hisDie Mutationstheorie, published in 1901, he argues for the recognition of mutations as the universal source of the origin of species. Although he evokes natural selection for the perpetuation and improvement of variations, and points out that his theory is not antagonistic to that of natural selection, it is nevertheless directly at variance with Darwin's fundamental conception—that slight individual variations "are probably the sole differences which are effective in the production of new species" and that "as natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modifications." The foundation of De Vries's theory is that "species have not arisen through gradual selection, continued for hundreds or thousands of years, but by jumps through sudden, through small transformations." (Whitman's translation.)
The work of De Vries is a most important contribution to the study of the origin of species, and is indicative of the fact that many factors must be taken into consideration when one attempts to analyze the process of organic evolution. One great value of his work is that it is based on experiments,and that it has given a great stimulus to experimental studies. Experiment was likewise a dominant feature in Darwin's work, but that seems to have been almost overlooked in the discussions aroused by his conclusions; De Vries, by building upon experimental evidence, has led naturalists to realize that the method of evolution is not a subject for argumentative discussion, but for experimental investigation. This is most commendable.
Fig. 115.—Hugo de Vries.
De Vries's theory tends also to widen the field of exploration. Davenport, Tower, and others have made it clear that species may arise by slow accumulations of trivial variations, and that, while the formation of species by mutationmay be admitted, there is still abundant evidence of evolution without mutation.
Reconciliation of Different Theories.—All this is leading to a clearer appreciation of the points involved in the discussion of the theories of evolution; the tendency is not for the breach between the different theories to be widened, but for evolutionists to realize more fully the great complexity of the process they are trying to explain, and to see that no single factor can carry the burden of an explanation. Mutation is not a substitute for natural selection, but a coöperating factor; and neither mutation nor natural selection is a substitute for the doctrine of the continuity of the germ-plasm. Thus we may look forward to a reconciliation between apparently conflicting views, when naturalists by sifting shall have determined the truth embodied in the various theories. One conviction that is looming into prominence is that this will be promoted by less argument and more experimental observation.
That the solution of the underlying question in evolution will still require a long time is evident; as Whitman said in his address before the Congress of Arts and Science in St. Louis in 1904: "The problem of problems in biology to-day, the problem which promises to sweep through the present century as it has the past one, with cumulative interest and correspondingly important results, is the one which became the life-work of Charles Darwin, and which can not be better or more simply expressed than in the title of his epoch-making book,The Origin of Species."
Summary.—The number of points involved in the four theories considered above is likely to be rather confusing, and we may now bring them into close juxtaposition. The salient features of these theories are as follows:
I. Lamarck's Theory of Evolution.