Fœtus of an Orang.Fig.21.—Fœtus of an Orang. Exact copy of a photograph, showing the form of the ear at this early stage.
Fig.21.—Fœtus of an Orang. Exact copy of a photograph, showing the form of the ear at this early stage.
The following woodcut serves still further to show vestigial resemblances between the human ear and that of apes. The last two figures illustrate the general resemblance between the normal ear of fœtal man and the ear of an adult orang-outang. The other two figures on the lower line are intended to exhibit occasional modifications of the adult human ear, which approximate simian characters somewhat more closely than does the normal type. It will be observed that in their comparatively small lobes these ears resemble those of all the apes; and that while the outer margin of one is not unlike that of the Barbaryape, the outer margin of the other follows those of the chimpanzee and orang. Of course it would be easy to select individual human ears which present either of these characters in a more pronounced degree; but these ears have been chosen as models because they present both characters in conjunction. The upper row of figures likewise shows the close similarity of hair-tracts, and the direction of growth on the part of the hair itself, in cases where the human ear happens to be of an abnormally hirsute character. But this particular instance (which I do not think has been previously noticed) introduces us to the subject of hair, and hair-growth, in general.
Vestigial characters of human ears.Fig.22.—Vestigial characters of human ears. Drawn from nature.
Fig.22.—Vestigial characters of human ears. Drawn from nature.
(8)Hair.—Adult man presents rudimentary hair over most parts of the body. Wallace has sought to draw a refined distinction between this vestigial coating and the useful coating of quadrumanous animals, in the absence of the former from the human back. But even this refined distinction does not hold. On the one hand, the comparatively hairless chimpanzee which died last year in the Zoological Gardens (T. calvus) was remarkably denuded over the back; and, on the other hand, men who present a considerable development of hair over the rest of their bodies present it also on their backs and shoulders. Again, in all men the rudimentary hair on the upper and lower arm is directed towards the elbow—a peculiarity which occurs nowhere else in the animal kingdom, with the exception of the anthropoid apes and a few American monkeys, where it presumably has to do with arboreal habits. For, when sitting in trees, the orang, as observed by Mr. Wallace, places its hands above its head with its elbows pointing downwards:the disposition of hair on the arms and fore-arms then has the effect of thatch in turning the rain. Again, I find that in all species of apes, monkeys, and baboons which I have examined (and they have been numerous), the hair on the backs of the hands and feet is continued as far as the first row of phalanges; but becomes scanty, or disappears altogether, on the second row; while it is invariably absent on the terminal row. I also find that the same peculiarity occurs in man. We all have rudimentary hair on the first row of phalanges, both of hands and feet: when present at all, it is more scanty on the second row; and in no case have I been able to find any on the terminal row. In all cases these peculiarities are congenital, and the total absence or partial presence of hair on the second phalanges is constant in different species of Quadrumana. For instance, it is entirely absent in all the chimpanzees, which I have examined, while scantily present in all the orangs. As in man, it occurs in a patch midway between the joints.
Hair-tracts on the arms and hands of Man and Chimpanzee.Fig.23.—Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee. Drawn from life.
Fig.23.—Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee. Drawn from life.
Besides showing these two features with regard to the disposition of hair on the human arm and hand, the above woodcut illustrates a third. By looking closely at the arm of the very hairy man from whom the drawing was taken, it could be seen that there was a strong tendency towards a whorled arrangement of the hairs on the backs of the wrists. This is likewise, as a general rule, a marked feature in the arrangement of hair on the same places in the gorilla, orang, and chimpanzee. In the specimen of the latter, however, from which the drawing was taken, this characteristic was not well marked. The downward direction of the hair on the backs of the handsis exactly the same in man as it is in all the anthropoid apes. Again, with regard to hair, Darwin notices that occasionally there appears in man a few hairs in the eyebrows much longer than the others; and that they seem to be representative of similarly long and scattered hairs which occur in the chimpanzee, macacus, and baboons.
Lastly, it may be here more conveniently observed than in the next chapter on Embryology, that at about the sixth month the human fœtus is often thickly coated with somewhat long dark hair over the entire body, except the soles of the feet and palms of the hands, which are likewise bare in all quadrumanous animals. This covering, which is called the lanugo, and sometimes extends even to the whole forehead, ears, and face, is shed before birth. So that it appears to be useless for any purpose other than that of emphatically declaring man a child of the monkey.
(9)Teeth.—Darwin writes:—
It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs.... They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound [i. e. not specially liable to decay]; they also differ from the other molars in size, less than in the Caucasian races.
It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs.... They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound [i. e. not specially liable to decay]; they also differ from the other molars in size, less than in the Caucasian races.
Now, in addition to these there are other respects in which the dwindling condition of wisdom-teeth is manifested—particularly with regard to the pattern oftheir crowns. Indeed, in this respect it would seem that even in the anthropoid apes there is the beginning of a tendency to degeneration of the molar teeth from behind forwards. For if we compare the three molars in the lower jaw of the gorilla, orang, and chimpanzee, we find that the gorilla has five well-marked cusps on all three of them; but that in the orang the cusps are not so pronounced, while in the chimpanzee there are only four of them on the third molar. Now in man it is only the first of these three teeth which normally presents five cusps, both the others presenting only four. So that, comparing allthese genera together, it appears that the number of cusps is being reduced from behind forwards; the chimpanzee having lost one of them from the third molar, while man has not only lost this, but also one from the second molar,—and, it may be added, likewise partially (or even totally) from the first molar, as a frequent variation among civilized races. But, on the other hand, variations are often met with in the opposite direction, where the second or the third molar of man presents five cusps—in the one case following the chimpanzee, in the other the gorilla. These latter variations, therefore, may fairly be regarded as reversionary. For these facts I am indebted to the kindness of Mr. C. S. Tomes.
Molar teeth of lower jaw in Gorilla, Orang, and Man.Fig.24.—Molar teeth of lower jaw in Gorilla, Orang, and Man. Drawn from nature, nat. size (R. Mus. Coll. Surg.).
Fig.24.—Molar teeth of lower jaw in Gorilla, Orang, and Man. Drawn from nature, nat. size (R. Mus. Coll. Surg.).
(10)Perforations of the humerus.—The peculiarities which we have to notice under this heading are two in number. First, the supra condyloid foramen is a normal feature in some of the lower Quadrumana (Fig. 25), where it gives passage to the great nerve of the fore-arm, and often also to the great artery. In man, however, it is not a normal feature. Yet it occurs in a small percentage of cases—viz., according to Sir W. Turner, in about one per cent., and therefore is regarded by Darwin as a vestigial character. Secondly, there is inter-condyloid foramen, which is also situated near the lower end of the humerus, but more in the middle of the bone. This occurs, but not constantly, in apes, and also in the human species. From the fact that it does so much more frequently in the bones of ancient—and also of some savage—races of mankind (viz. in 20 to 30 per cent. of cases), Darwin is disposed to regard it also as a vestigial feature. On the other hand, Prof. Flower tells me that in his opinion it is but an expressionof impoverished nutrition during the growth of the bone.
Perforation of the humerus in three species of Quadrumana and in Man.Fig.25.—Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur. Drawn from nature (R. Coll. Surg. Mus.).
Fig.25.—Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur. Drawn from nature (R. Coll. Surg. Mus.).
(11)Flattening of tibia.—In some very ancient human skeletons, there has also been found a lateral flattening of the tibia, which rarely occurs in any existing human beings, but which appears to have been usual among the earliest races of mankind hitherto discovered. According to Broca, the measurements of these fossil human tibiæ resemble those of apes. Moreover, the bone is bent and strongly convex forwards, while its angles are so rounded as to present the nearly oval section seen in apes. It is in association with these ape-like human tibiæ that perforated humeri of man are found in greatest abundance.
On the other hand, however, there is reason to doubt whether this form of tibia in man is really a survival from his quadrumanous ancestry. For, as Boyd-Dawkins and Hartmann have pointed out, the degree of flattening presented by some of these ancient human bones isgreaterthan that which occurs in any existing species of anthropoid ape. Of course the possibility remains that the unknown species of ape from which man descended may have had its tibia more flattened than is now observable in any of the existing species. Nevertheless, as some doubt attaches to this particular case, I do not press it—and, indeed, only mention it at all in order that the doubt may be expressed.
Similarly, I will conclude by remarking that several other instances of the survival of vestigial structures in man have been alleged, which are of a still more doubtful character. Of such, for example, are the supposed absence of the genial tubercle in the case of a very ancient jaw-bone of man, and the disposition of valves in human veins. From the former it was argued that the possessor of this very ancient jaw-bone was probably speechless, inasmuch as the tubercle in existing man gives attachment to muscles of the tongue. From the latter it has been argued that all the valves in the veins of the human body have reference, in their disposition, to the incidence of blood-pressure when the attitude of the body is horizontal, or quadrupedal. Now, the former case has already broken down, and I find that the latter does not hold. But we can well afford to lose such doubtful and spurious cases, in view of all the foregoing unquestionable and genuine cases of vestigial structures which areto be met with even within the limits of our own organization—and even when these limits are still further limited by selecting only those instances which refer to the very latest chapter of our long ancestral history.
We will next consider what of late years has become the most important of the lines of evidence, not only in favour of the general fact of evolution, but also of its history: I mean the evidence which has been yielded by the newest of the sciences, the science of Embryology. But here, as in the analogous case of adult morphology, in order to do justice to the mass of evidence which has now been accumulated, a whole volume would be necessary. As in that previous case, therefore, I must restrict myself to giving an outline sketch of the main facts.
First I will display what in the language of Paley we may call “the state of the argument.”
It is an observable fact that there is often a close correspondence between developmental changes as revealed by any chronological series of fossils which may happen to have been preserved, and developmental changes which may be observed during the life-history of now existing individuals belonging to the same group of animals. For instance, the successive development of prongs in the horns of deer-like animals, which is so clearly shown in the geological history of this tribe, is closely reproducedin the life-history of existing deer. Or, in other words, the antlers of an existing deer furnish in their development a kind ofrésumé, or recapitulation, of the successive phases whereby the primitive horn was gradually superseded by horns presenting a greater and greater number of prongs in successive species of extinct deer (Fig. 26). Now it must be obvious that such a recapitulation in the life-history of an existing animal of developmental changes successively distinctive of sundry allied, though now extinct species, speaks strongly in favour of evolution. For as it is of the essence of this theory that new forms arise from older forms by way ofhereditarydescent, we should antecedently expect, if the theory is true, that the phases of development presented by the individual organism would follow, in their main outlines, those phases of development through which their long line of ancestors had passed. The only alternative view is that as species of deer, for instance, were separately created, additional prongs were successively added to their antlers; and yet that, in order to be so added to successive species every individual deer belonging to later species was required to repeat in his own lifetime the process of successive additions which had previously taken place in a remote series of extinct species. Now I do not deny that this view is a possible view; but I do deny that it is a probable one. According to the evolutionary interpretation of such facts, we can see a very goodreasonwhy the life-history of the individual is thus a condensedrésuméof the life-history of its ancestral species. But according to the opposite view no reason can be assigned why such should be the case. In a previous chapter—thechapter on Classification—we have seen that if each species were created separately, no reason can be assigned why they should all have been turned out upon structural patterns so strongly suggestive of hereditary descent with gradual modifications, or slow divergence—the result being group subordinated to group, with the most generalized (or least developed) forms at the bottom, and the highest products of organization at the top. And now we see—or shall immediately see—that this consideration admits of being greatly fortified by a study of the developmental history of every individual organism. If it would be an unaccountable fact that every separately created species should have been created with close structural resemblances to a certain limited number of other species, less close resemblances to certain further species, and so backwards; assuredly it would be a still more unaccountable fact that every individual of every species should exhibit in its own person a history of developmental change, every term of which corresponds with the structural peculiarities of its now extinct predecessors—and this in the exact historical order of their succession in geological time. The more that we think about this antithesis between the naturalistic and the non-naturalistic interpretations, the greater must we feel the contrast in respect of rationality to become; and, therefore, I need not spend time by saying anything further upon the antecedent standing of the two theories in this respect. The evidence, then, which I am about to adduce from the study of development in the life-histories of individual organisms, will be regarded by me as so much unquestionable evidence in favour ofsimilar processes of development in the life-histories of their respective species—in so far, I mean, as the two sets of changes admit of being proved parallel.
Antlers of Stag.Fig.26.—Antlers of Stag, showing successive addition of branches in successive years. Drawn from nature (Brit. Mus.).
Fig.26.—Antlers of Stag, showing successive addition of branches in successive years. Drawn from nature (Brit. Mus.).
In the only illustration hitherto adduced—viz. that of deers’ horns—the series of changes from a one-pronged horn to a fully developed arborescent antler, is a series which takes place during the adult life of the animal; for it is only when the breeding age has been attained that horns are required to appear. But seeing that every animal passes through most of the phases of its development, not only before the breeding age has been attained, but even before the time of its own birth, clearly the largest field for the study of individual development is furnished by embryology. For instance, there is a salamander which differs from most other salamanders in being exclusively terrestrial in its habits. Now, the young of this salamander before their birth are found to be furnished with gills, which, however, they are never destined to use. Yet these gills are so perfectly formed, that if the young salamanders be removed from the body of their mother shortly before birth, and be then immediately placed in water, the little animals show themselves quite capable of aquatic respiration, and will merrily swim about in a medium which would quickly drown their own parent. Here, then, we have both morphological and physiological evidence pointing to the possession of gills by the ancestors of the land salamander.
It would be easy to devote the whole of the present chapter to an enumeration of special instances of the kinds thus chosen for purposes of illustration; but as it is desirable to take a deeper, and thereforea more general view of the whole subject, I will begin at the foundation, and gradually work up from the earliest stages of development to the latest. Before starting, however, I ask the reader to bear in mind one consideration, which must reasonably prevent our anticipating that inevery casethe life-history of an individual organism should present afullrecapitulation of the life-history of its ancestral line of species. Supposing the theory of evolution to be true, it must follow that in many cases it would have been more or less disadvantageous to a developing type that it should have been obliged to reproduce in its individual representatives all the phases of development previously undergone by its ancestry—even within the limits of the same family. We can easily understand, for example, that the waste of material required for building up the useless gills of the embryonic salamanders is a waste which, sooner or later, is likely to be done away with; so that the fact of its occurring at all is in itself enough to show that the change from aquatic to terrestrial habits on the part of this species must have been one of comparatively recent occurrence. Now, in as far as it is detrimental to a developing type that it should pass through any particular ancestral phases of development, we may be sure that natural selection—or whatever other adjustive causes we may suppose to have been at work in the adaptation of organisms to their surroundings—will constantly seek to get rid of this necessity, with the result, when successful, of dropping out the detrimental phases. Thus the foreshortening of developmental history which takes place in the individual lifetime may be expected often to take place, not only in the way ofcondensation, but also in the way of excision. Many pages of ancestral history may be recapitulated in the paragraphs of embryonic development, while others may not be so much as mentioned. And that this is the true explanation of what embryologists term “direct” development—or of a more or less sudden leap from one phase to another, without any appearance of intermediate phases—is proved by the fact that in some cases both direct and indirect development occur within the same group of organisms, some genera or families having dropped out the intermediate phases which other genera or families retain.
The argument from embryology must be taken to begin with the first beginning of individual life in the ovum. And, in order to understand the bearings of the argument in this its first stage, we must consider the phenomena of reproduction in the simplest form which these phenomena are known to present.
The whole of the animal kingdom is divided into two great groups, which are called the Protozoa and the Metazoa. Similarly, the whole of the vegetable kingdom is divided into the Protophyta and the Metaphyta. The characteristic feature of all the Protozoa and Protophyta is that the organism consists of a single physiological cell, while the characteristic of all the Metazoa and Metaphyta is that the organism consists of a plurality of physiological cells, variously modified to subserve different functions in the economy of the animal or plant, as the case may be. For the sake of brevity, I shall hereafter deal only with the case of animals (Protozoa and Metazoa); but it may throughout be understood that everythingwhich is said applies also to the case of plants (Protophyta and Metaphyta).
A Protozoön (like a Protophyton) is a solitary cell, or a “unicellular organism,” while a Metazoön (like a Metaphyton) is a society of cells, or a “multicellular organism.” Now, it is only in the multicellular organisms that there is any observable distinction of sex. In all the unicellular organisms the phenomena of reproduction appear to be more or less identical with those of growth. Nevertheless, as these phenomena are here in some cases suggestively peculiar, I will consider them more in detail.
A Protozoön is a single corpuscle of protoplasm which in different species of Protozoa varies in size from more than one inch to less than 1/1000 of an inch in diameter. In some species there is an enveloping cortical substance; in other species no such substance can be detected. Again, in most species there is a nucleus, while in other species no such differentiation of structure has hitherto been observed. Nevertheless, from the fact that the nucleus occurs in the majority of Protozoa, coupled with the fact that the demonstration of this body is often a matter of extreme difficulty, not only in some of the Protozoa where it has been but recently detected, but also in the case of certain physiological cells elsewhere,—from these facts it is not unreasonable to suppose that all the Protozoa possess a nucleus, whether or not it admits of being rendered visible by histological methods thus far at our disposal. If this is the case, we should be justified in saying, as I have said, that a Protozoön is an isolated physiological cell, and, like cells in general, multiplies by means of what Spencer and Häckelhave aptly called a process of discontinuous growth. That is to say, when a cell reaches maturity, further growth takes place in the direction of a severance of its substance—the separated portion thus starting anew as a distinct physiological unit. But, notwithstanding the complex changes which have been more recently observed to take place in the nucleus of some Protozoa prior to their division, the process of multiplication by division may still be regarded as a process of growth, which differs from the previous growth of the individual cell in being attended by a severance of continuity. If we take a suspended drop of gum, and gradually add to its size by allowing more and more gum to flow into it, a point will eventually be reached at which the force of gravity will overcome that of cohesion, and a portion of the drop will fall away from the remainder. Here we have a rough physical simile, although of course no true analogy. In virtue of a continuous assimilation of nutriment, the protoplasm of a cell increases in mass, until it reaches the size at which the forces of disruption overcome those of cohesion—or, in other words, the point at which increase of size is no longer compatible with continuity of substance. Nevertheless, it must not be supposed that the process is thus merely a physical one. The phenomena which occur even in the simplest—or so-called “direct"—cell-division, are of themselves enough to prove that the process is vital, or physiological; and this in a high degree of specialization. But so, likewise, are all processes of growth in organic structures; and therefore the simile of the drop of gum is not to be regarded as a true analogy: it serves only toindicate the fact that when cell-growth proceeds beyond a certain point cell-division ensues. The size to which cells may grow before they thus divide is very variable in different kinds of cells; for while some may normally attain a length of ten or twelve inches, others divide before they measure 1/1000 of an inch. This, however, is a matter of detail, and does not affect the general physiological principles on which we are at present engaged.
Now, as we have seen, a Protozoön is a single cell; for even although in some of the higher forms of protozoal life a colony of cells may be bound together in organic connexion, each of these cells is in itself an “individual,” capable of self-nourishment, reproduction, and, generally, of independent existence. Consequently, when the growth of a Protozoön ends in a division of its substance, the two parts wander away from each other as separate organisms. (Fig. 27.)
Fission of a Protozoön.Fig.27.—Fission of a Protozoön. In the left-hand drawing the process is represented as having advanced sufficiently far to have caused a division and segregation both of the nucleus and the vesicle. In the right-hand drawing the process is represented as complete.n, N, severed nucleus;vc, severed vesicle;ps, pseudopodia;f, ingested food.
Fig.27.—Fission of a Protozoön. In the left-hand drawing the process is represented as having advanced sufficiently far to have caused a division and segregation both of the nucleus and the vesicle. In the right-hand drawing the process is represented as complete.n, N, severed nucleus;vc, severed vesicle;ps, pseudopodia;f, ingested food.
The next point we have to observe is, that in all cases where a cell or a Protozoön multiplies by way of fissiparous division, the process begins in the nucleus. If the nucleus divides into two parts, the whole cell will eventually divide into two parts, each of which retains a portion of the original nucleus, as represented in the above figure. If the nucleus divides into three, four, or even, as happens in the development of some embryonic tissues, into as many as six parts, the cell will subdivide into a corresponding number, each retaining a portion of the nucleus. Therefore, in all cases of fissiparous division, the seat or origin of the process is the nucleus.
Thus far, then, the phenomena of multiplication are identical in all the lowest or unicellular organisms, and in the constituent cells of all the higher or multicellular. And this is the first point which I desire to make apparent. For where the object is to prove a continuity between the phenomena of growth and reproduction, it is of primary importance to show—1st, that there is such a continuity in the case of all the unicellular organisms, and, 2nd, that there are all the above points of resemblance between the multiplication of cells in the unicellular and in the multicellular organisms.
It remains to consider the points of difference, and, if possible, to show that these do not go to disprove the doctrine of continuity which the points of resemblance so forcibly indicate.
The first point of difference obviously is, that in the case of all the multicellular organisms the two or more “daughter-cells,” which are produced by division of the “mother-cell,” do not wander away from oneanother; but, as a rule, they continue to be held in more or less close apposition by means of other cells and binding membranes,—with the result of giving rise to those various “tissues,” which in turn go to constitute the material of “organs.” I cannot suppose, however, that any advocate of discontinuity will care to take his stand at this point. But, if any one were so foolish as to do so, it would be easy to dislodge him by describing the state of matters in some of the Protozoa where a number of unicellular “individuals” are organically united so as to form a “colony.” These cases serve to bridge this distinction between Protozoa and Metazoa, of which therefore we may now take leave.
In the second place, there is the no less obvious distinction that the result of cell-division in the Metazoa is not merely to multiply cells all of the same kind: on the contrary, the process here gives rise to as many different kinds of cells as there are different kinds of tissue composing the adult organism. But no one, I should think, is likely to oppose the doctrine of continuity on the ground of this distinction. For the distinction is clearly one which must necessarily arise, if the doctrine of continuity between unicellular and multicellular organisms be true. In other words, it is a distinction which the theory of evolution itself must necessarily pre-suppose, and therefore it is no objection to the theory that its pre-supposition is realized. Moreover, as we shall see better presently, there is no difficulty in understanding why this distinction should have arisen, so soon as it became necessary (or desirable) that individual cells, when composing a “colony,” shouldconform to the economic principle of the division of labour—a principle, indeed, which is already foreshadowed in the constituent parts of a single cell, since the nucleus has one set of functions and its surrounding protoplasm another.
But now, in the third place, we arrive at a more important distinction, and one which lies at the root of the others still remaining to be considered. I refer to sexual propagation. For it is a peculiarity of the multicellular organisms that, although many of them may likewise propagate themselves by other means (Fig. 28), they all propagate themselves by means of sexual congress. Now, in its essence, sexual congress consists in the fusion of two specialized cells (or, as now seems almost certain, of the nuclei thereof), so that it is out of such a combination that the new individual arises by means of successive cell-divisions, which, beginning in the fertilized ovum, eventually build up all the tissues and organs of the body.
Hydra viridis.Fig.28.—Hydra viridis, partly in section. M, mouth; O, ovary, or bud containing female reproductive cells; T, testis, or bud containing male reproductive cells. In addition to these buds containing germinal elements alone, there is another which illustrates the process of “gemmation"—i. e. the direct out-growth of a fully formed offspring.
Fig.28.—Hydra viridis, partly in section. M, mouth; O, ovary, or bud containing female reproductive cells; T, testis, or bud containing male reproductive cells. In addition to these buds containing germinal elements alone, there is another which illustrates the process of “gemmation"—i. e. the direct out-growth of a fully formed offspring.
This process clearly indicates very high specialization on the part of germ-cells. For we see by it that although these cells when young resemble all other cells in being capable of self-multiplication by binary division (thus reproducing cells exactly like themselves), when older they lose this power; but, at the same time, they acquire an entirely new and very remarkable power of giving rise to a vast succession of many different kinds of cells, all of which are mutually correlated as to their several functions, so as to constitute a hierarchy of cells—or, to speak literally, a multicellularco-organization. Here it is that we touch the really important distinction between the Protozoa and the Metazoa; for although I havesaid that some of the higher Protozoa foreshadow this state of matters in forming cell-colonies, it must now be noted that the cells composing such colonies are all of the same kind; and, therefore, that the principle of producing different kinds of cells which, by mutual co-adaptation of functions, shall be capable of constructing a multicellular Metazoön,—this great principle ofco-organizationis but dimly nascent in the cell-coloniesof Protozoa. And its marvellous development in the Metazoa appears ultimately to depend upon the highly specialized character of germ-cells. Even in cases where multicellular organisms are capable of reproducing their kind without the need of any preceding process of fertilization (parthenogenesis), and even in the still more numerous cases where complete organisms are budded forth from any part of their parent organism (gemmation, Fig. 28), there is now very good reason to conclude that these powers of a-sexual reproduction on the part of multicellular organisms are all ultimately due to the specialized character of their germ-cells. For in all these cases the tissues of the parent, from which the budding takes place, were ultimately derived from germ-cells—no matter how many generations of budded organisms may have intervened. And that propagation by budding, &c., in multicellular organisms is thus ultimately due to their propagation by sexual methods, seems to be further shown by certain facts which will have to be discussed at some length in my next volume. Here, therefore, I will mention only one of them—and this because it furnishes what appears to be another important distinction between the Protozoa and the Metazoa.
In nearly all cases where a Protozoön multiplies itself by fission, the process begins by a simple division of the nucleus. But when a Metazoön is developed from a germ-cell, although the process likewise begins by a division of the nucleus, this division is not a simple or direct one; on the contrary, it is inaugurated by a series of processes going on within the nucleus, which are so enormously complex, and withal sobeautifully ordered, that to my mind they constitute the most wonderful—if not also the most suggestive—which have ever been revealed by microscopical research. It is needless to say that I refer to the phenomena of karyokinesis. A few pages further on they will be described more fully. For our present purposes it is sufficient to give merely a pictorial illustration of their successive phases; for a glance at such a representation serves to reveal the only point to which attention has now to be drawn—namely, the immense complexity of the processes in question, and therefore the contrast which they furnish to the simple (or “direct") division of the nucleus preparatory to cell-division in the unicellular organisms. Here, then(Fig. 29), we see the complex processes of karyokinesis in the first two stages of egg-cell division. But similar processes continue to repeat themselves in subsequent stages; and this, there is now good reason to believe, throughoutallthe stages of cell-division, whereby the original egg-cell eventually constructs an entire organism. In other words, all the cells composing all the tissues of a multicellular organism, at all stages of its development, are probably originated by these complex processes, which differ so much from the simple process of direct division in the unicellular organisms[9]. In this important respect, therefore, it does at first sight appear that we have a distinction between the Protozoa and the Metazoa of so pronounced a character, as fairly to raise the question whether cell-division is fundamentally identical in unicellular and in multicellular organisms.
Stages in the division of the ovum of a worm.Fig.29.—Successive stages in the division of the ovum, or egg-cell, of a worm. (After Strasburger.)atodshow the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum ate;fandgshow a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage ath.
Fig.29.—Successive stages in the division of the ovum, or egg-cell, of a worm. (After Strasburger.)atodshow the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum ate;fandgshow a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage ath.
Lastly, the only other distinction of a physiologically significant kind between a single cell when it occurs as a Protozoön and when it does so as the unfertilized ovum of a Metazoön is, that in the latter case the nucleus discharges from its own substance two minute protoplasmic masses ("polar bodies"), which are then eliminated from the cell altogether. This process, which will be more fully described later on, appears to be of invariable occurrence in the case of all egg-cells,while nothing resembling it has ever been observed in any of the Protozoa.
We must now consider these several points of differenceseriatim.
First, with regard to sexual propagation, we have already seen that this is by no means the only method of propagation among the multicellular organisms; and it now remains to add that, on the other hand, there is, to say the least, a suggestive foreshadowing of sexual propagation among the unicellular organisms. For although simple binary fission is here the more usual mode of multiplication, very frequently two (rarely three or more) Protozoa of the same species come together, fuse into a single mass, and thus become very literally “one flesh.” This process of “conjugation” is usually (though by no means invariably) followed by a period of quiescent “encystation"; after which the contents of the cyst escape in the form of a number of minute particles, or “spores,” and these severally develope into the parent type. Obviously this process of conjugation, when it is thus a preliminary to multiplication, appears to be in its essence the same as fertilization. And if it be objected that encystation and spore-formation in the Protozoa are not always preceded by conjugation, the answer would be that neither is oviparous propagation in the Metazoa invariably preceded by fertilization.
Nevertheless, that there are great distinctions between true sexual propagation and this foreshadowing of it in conjugation I do not deny. The question, however, is whether they be so great as to justify any argument against an historical continuity between them. What, then, are these remainingdistinctions? Briefly, as we have seen, they are the extrusion from egg-cells of polar bodies, and the occurrence, both in egg-cells and their products (tissue-cells), of the process of karyokinesis. But, as regards the polar bodies, it is surely not difficult to suppose that, whatever their significance may be, it is probably in some way or another connected with the high specialization of the functions which an egg-cell has to discharge. Nor is there any difficulty in further supposing that, whatever purpose is served by getting rid of polar bodies, the process whereby they are got rid of was originally one of utilitarian development—i. e. a process which at its commencement did not betoken any difference of kind, or breach of continuity, between egg-cells and cells of simpler constitution.
Lastly, with respect to karyokinesis, although it is true that the microscope has in comparatively recent years displayed this apparently important distinction between unicellular and multicellular organisms, two considerations have here to be supplied. The first is, that in some of the Protozoa processes very much resembling those of karyokinesis have already been observed taking place in the nucleus preparatory to its division. And although such processes do not present quite the same appearances as are to be met with in egg-cells, neither do the karyokinetic processes in tissue-cells, which in their sundry kinds exhibit great variations in this respect. Moreover, even if such were not the case, the bare fact that nuclear division is not invariably of the simple or direct character in the case of all Protozoa, is sufficient to show that the distinction now before us—like the one last dealt with—is by no meansabsolute. As in the case of sexual propagation, so in that of karyokinesis, processes which are common to all the Metazoa are not wholly without their foreshadowings in the Protozoa. And seeing how greatly exalted is the office of egg-cells—and even of tissue-cells—as compared with that of their supposed ancestry in protozoal cells, it seems to me scarcely to be wondered at if their specializations of function should be associated with corresponding peculiarities of structure—a general fact which would in no way militate against the doctrine of evolution. Could we know the whole truth, we should probably find that in order to endow the most primitive of egg-cells with its powers of marshalling its products into a living army of cell-battalions, such an egg-cell must have been passed through a course of developmental specialization of so elaborate a kind, that even the complex processes of karyokinesis are but a very inadequate expression thereof.
Probably I have now said enough to show that, remarkable and altogether exceptional as the properties of germ-cells of the multicellular organisms unquestionably show themselves to be, yet when these properties are traced back to their simplest beginnings in the unicellular organisms, they may fairly be regarded as fundamentally identical with the properties of living cells in general. Thus viewed, no line of real demarcation can be drawn between growth and reproduction, even of the sexual kind. The one process is, so to speak, physiologically continuous with the other; and hence, so far as the pre-embryonic stage of life-history is concerned, the facts cannot fairly be regarded as out of keeping with the theory of evolution.
I will now pass on to consider the embryogeny of the Metazoa, beginning at its earliest stage in the fertilization of the ovum. And here it is that the constructive argument in favour of evolution which is derived from embryology may be said properly to commence. For it is surely in itself a most suggestive fact that all the Metazoa begin their life in the same way, or under the same form and conditions.Omne vivum ex ovo.This is a formula which has now been found to apply throughout the whole range of the multicellular organisms. And seeing, as we have just seen, that the ovum is everywhere a single cell, the formula amounts to saying that, physiologically speaking, every Metazoön begins its life as a Protozoön, and every Metaphyton as a Protophyton[10].
Now, if the theory of evolution is true, what should we expect to happen when these germ-cells are fertilized, and so enter upon their severally distinct processes of development? Assuredly we should expect to find that the higher organisms pass through the same phases of development as the lower organisms, up to the time when their higher characters begin to become apparent. If in the life-history of species these higher characters were gained by gradual improvement upon lower characters, and if the development of the higher individual is now a general recapitulation of that of its ancestral species, in studying this recapitulation we should expect to find the higher organism successively unfolding its higher characters from the lower ones through which its ancestral species had previously passed. And this is just what we dofind. Take, for example, the case of the highest organism, Man. Like that of all other organisms, unicellular or multicellular, his development starts from the nucleus of a single cell. Again, like that of all the Metazoa and Metaphyta, his development starts from the specially elaborated nucleus of an egg-cell, or a nucleus which has been formed by the fusion of a male with a female element[11]. When his animality becomes established, he exhibits the fundamental anatomical qualities which characterize such lowly animals as polyps and jelly-fish. And even when he is marked off as a Vertebrate, it cannot be said whether he is to be a fish, a reptile, a bird, or a beast. Later on it becomes evident that he is to be a Mammal; but not till later still can it be said to which order of mammals he belongs.
Here, however, we must guard against an error which is frequently met with in popular expositions of this subject. It is not true that the embryonic phases in the development of a higher form always resemble so many adult stages of lower forms. This may or may not be the case; but what always is the caseis, that the embryonic phases of the higher form resemble the corresponding phases of the lower forms. Thus, for example, it would be wrong to suppose that at any stage of his development a man resembles a jelly-fish. What he does resemble at an early stage of his development is the essential or groundplan of the jelly-fish, which that animal presents initsembryonic condition, or before it begins to assume its more specialized characters fitting it for its own particular sphere of life. The similarities, therefore, which it is the function of comparative embryology to reveal are the similarities of type or morphological plan: not similarities of specific detail. Specific details may have been added to this, that, and the other species for their own special requirements, after they had severally branched off from the common ancestral stem; and so could not be expected to recur in the life-history of an independent specific branch. The comparison therefore must be a comparison of embryo with embryo; not of embryos with adult forms.
In order to give a general idea of the results thus far yielded by a study of comparative embryology in the present connexion, I will devote the rest of this chapter to giving an outline sketch of the most important and best established of these results.
Histologically the ovum, or egg-cell, is nearly identical in all animals, whether vertebrate or invertebrate. Considered as a cell it is of large size, but actually it is not more than 1/100, and may be less than 1/200 of an inch in diameter. In man, as in most mammals, it is about 1/120. It is a more or less spherical body, presenting a thin transparent envelope, calledthezona pellucida, which contains—first, the protoplasmic cell-substance or “yolk,” within which lies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. This description is true of the egg-cells of all animals, if we add that in the case of the lowest animals—such as sponges, &c.—there is no enveloping membrane: the egg-cell is here a naked cell, and its constituent protoplasm, being thus unconfined, is free to perform protoplasmic movements, which it does after the manner, and with all the activity, of an amœba. But even with respect to this matter of an enveloping membrane, there is no essential difference between an ovum of the lowest and an ovum of the highest animals. For in their early stages of development within the ovary the ova of the highest animals are likewise in the condition of naked cells, exhibiting amœbiform movements; the enveloping membrane of an ovum being the product of a later development.Moreover this membrane, when present, is usually provided with one or more minute apertures, through which the spermatozoön passes when fertilizing the ovum. It is remarkable that the spermatozoa know, so to speak, of the existence of these gate-ways,—their snake-like movements being directed towards them, presumably by a stimulus due to some emanation therefrom[12]. In the mammalian ovum, however, these apertures are exceedingly minute, and distributedall round the circumference of the pellucid envelope, as represented in this illustration (Fig. 32).