First, the mandibulate;Secondly, the suctorial; andThirdly, that ofCampodeaand the Collembola generally,
First, the mandibulate;
Secondly, the suctorial; and
Thirdly, that ofCampodeaand the Collembola generally,
in which the mandibles and maxillæ are retracted, but have some freedom of motion, and can be used for biting and chewing soft substances. This type is, in some respects, intermediate between the other two. Assuming that certain representatives of such a type were placed under conditions which made a suctorial mouth advantageous, those individuals in which the mandibles and maxillæ were best calculated to pierce or prick would be favoured by natural selection, and their power of lateral motion would tend to fall intoabeyance; while, on the other hand, if masticatory jaws were an advantage, the opposite process would take place.
There is yet a third possibility—namely, that during the first portion of life, the power of mastication should be an advantage, and during the second that of suction, orvice versâ. A certain kind of food might abound at one season and fail at another; might be suitable for the animal at one age and not at another. Now in such cases we should have two forces acting successively on each individual, and tending to modify the organization of the mouth in different directions. It cannot be denied that the innumerable variations in the mouth-parts of insects have special reference to their mode of life, and are of some advantage to the species in which they occur. Hence, no believer in natural selection can doubt the possibility of the three cases above suggested, the last of which seems to throw some light on the possible origin of species which are mandibulate in one period of life and not in another. Granting then the transition from the one condition to the other, this would no doubt take place contemporaneously with a change of skin. At such times we know that, even when there is no change in form, the softness of the organs temporarily precludes the insect from feeding for a time, as, for instance, in the case of caterpillars. If, however, any considerable change were involved, this period of fasting must be prolonged, and would lead to the existence of a third condition, that of the pupa, intermediate between the other two. Since the acquisition of wings is a more conspicuouschange than any relating to the mouth, we are apt to associate with it the existence of a pupa-state: but the case of the Orthoptera (grasshoppers, &c.) is sufficient proof that the development of wings is perfectly compatible with permanent activity; the necessity for prolonged rest is in reality much more intimately connected with the change in the constitution of the mouth, although in many cases, no doubt, this is accompanied by changes in the legs, and in the internal organization. An originally mandibulate mouth, however, like that of a beetle, could not, I think, have been directly modified into a suctorial organ like that of a butterfly or a gnat, because the intermediate stages would necessarily be injurious. Neither, on the other hand, for the same reasons, could the mouth of the Hemiptera be modified into a mandibulate type like that of the Coleoptera. But inCampodeaand theCollembolawe have a type of animal closely resembling certain larvæ which occur both in the mandibulate and suctorial series of insects, possessing a mouth neither distinctly mandibulate nor distinctly suctorial, but constituted on a peculiar type, capable of modification in either direction by gradual change, without loss of utility.
In discussing this subject, it is necessary also to take into consideration the nature and origin of wings. Whence are they derived? why are there normally two pairs? and why are they attached to the meso-and meta-thorax? These questions are as difficult as they are interesting. It has been suggested, and I think with justice, that the wings of insects originally served for aquatic and respiratory purposes.
In the larva ofChloëon(Pl.IV., Fig. 1), for instance, which in other respects so singularly resemblesCampodea(Pl.III., Fig. 5), several of the segments are provided with foliaceous expansions which serve as respiratory organs. These so-called branchiæ are in constant agitation, and the muscles which move them in several points resemble those of true wings. It is true that inChloëonthe vibration of the branchiæ is scarcely, if at all, utilized for the purpose of locomotion; the branchiæ are, in fact, placed too far back to act efficiently. The situation of these branchiæ differs in different groups; indeed, it seems probable that originally there were a pair on each segment. In such a case, those branchiæ situated near the centre of the body, neither too much in front nor too far back, would serve the most efficiently as propellers: the same causes which determined the position of the legs would also affect the wings. Thus a division of labour would be effected; the branchiæ on the thorax would be devoted to locomotion; those on the abdomen to respiration. This would tend to increase the development of the thoracic segments, already somewhat enlarged, in order to receive the muscles of the legs.
That wings may be of use to insects under water is proved by the very interesting case ofPolynema natans,46which uses its wings for swimming. This, however, is a rare case, and it is possible that the principal use of the wings was, primordially, to enable the mature forms to pass from pond to pond, thus securing fresh habitats and avoiding in-and-inbreeding. If this were so, the development of wings would gradually have been relegated to a late period of life; and by the tendency to the inheritance of characters at corresponding ages, which Mr. Darwin has pointed out,47the development of wings would have thus become associated with the maturity of the insect. Thus the late acquisition of wings in the Insecta generally seems to be itself an indication of their descent from a stock which was at one period, if not originally, aquatic, and which probably resembled the present larvæ ofChloëonin form, but had thoracic as well as abdominal branchiæ.
Finally, from the subject of metamorphosis we pass naturally to that most remarkable phenomenon which is known as the “Alternation of Generations:” for the first systematic view of which we are indebted to my eminent friend Prof Steenstrup.48
I have always felt it very difficult to understand why any species should have been created in this double character; nor, so far as I am aware, has any explanation of the fact yet been attempted. Nevertheless insects offer, in their metamorphoses, a phenomenon not altogether dissimilar, and give a clue to the manner in which alternation of generations may have originated.
The caterpillar owes its difference from the butterfly to the undeveloped state in which it leaves the egg; but its actual form is mainly due to the influence of the conditions under which it lives. If the caterpillar, instead of changing into one butterfly, produced several, we should have an instance of alternation of generations. Until lately, however, we knew of no such case among insects; each larva produced one imago, and that not by generation, but by development. It has long been known, indeed, that there are species in which certain individuals remain always apterous, while others acquire wings. Many entomologists, however, regard these abnormal individuals as perfect, though wingless insects; and therefore I shall found no argument upon these cases, although they appear to me deserving of more attention than they have yet received.
Recently, however, Prof. Wagner49has discovered that, among certain small gnats, the larvæ do not directly produce in all cases perfect insects, but give birth to other larvæ, which undergo metamorphoses of the usual character, and eventually become gnats. His observations have been confirmed, as regards this main fact, by other naturalists; and Grimm has met with a species ofChironomusin which the pupæ lay eggs.50
Here, then, we have a distinct case of alternation of generations, as characterized by Steenstrup. Probably other cases will be discovered in which insects undeniably in the larval state will be found fertile. Nay, it seems to me possible, if not probable, that some larvæ which do not now breed may, in the course of ages, acquire the power of doing so. If this idea is correct, it shows how the remarkable phenomenon,known as alternation of generations, may have originated.
Summing up, then, the preceding argument, we find among insects various modes of development; from simple growth on the one hand, to well-marked instances of the so-called alternation of generation on the other. In the wingless species of Orthoptera there is little external difference, excepting in size, between the young larva and the perfect insect. The growth is gradual, and there is nothing which would, in ordinary language, be called a metamorphosis. In the majority of Orthoptera, though the presence of wings produces a marked difference between the larva and the imago, the habits are nearly the same throughout life, and consequently the action of external circumstances affects the larva in the same manner as it does the perfect insect.
This is not the case with the Neuroptera. The larvæ do not live under the same conditions as the perfect insects: external forces accordingly affect them in a different manner; and we have seen that they pass through some changes which bear no reference to the form of the perfect insect: these changes, however, are for the most part very gradual. The caterpillars of Lepidoptera have even more extensive modifications to undergo; the mouth of the larva, for instance, being remarkably unlike that of the perfect insect. A change in this organ, however, could hardly take place while the insect was growing fast, and consequently feeding voraciously; nor, even if the change could be thus effected, would the mouth, in its intermediate stages, be in any way fittedfor biting and chewing leaves. The same reasoning applies also to the digestive organs. Hence the caterpillar undergoes little, if any, change, except in size, and the metamorphosis is concentrated, so to say, into the last two moults. The changes then become so rapid and extensive, that the intermediate period is necessarily one of quiescence. In some exceptional cases, as inSitaris(ante, p. 30) we even find that, the conditions of life not being uniform throughout the larval period, the larva itself undergoes metamorphoses.
Owing to the fact that the organs connected with the reproduction of the species come to maturity at a late period, larvæ are generally incapable of breeding. There are, however, some flies which have viviparous larvæ, and thus offer a typical case of alternation of generations.
Thus, then, we find among insects every gradation, from simple growth to alternation of generations; and see how, from the single fact of the very early period of development at which certain animals quit the egg, we can throw some light on their metamorphoses, and for the still more remarkable phenomenon that, among many of the lower animals, the species is represented by two very different forms. We may even conclude, from the same considerations, that this phenomenon may in the course of ages become still more common than it is at present. As long, however, as the external organs arrive at their mature form before the internal generative organs are fully developed, we have metamorphosis; but if the reverse is the case, then alternation of generations often results.
The same considerations throw much light on the remarkable circumstance, that in alternation of generations the reproduction is, as a general rule, agamic in one form. This results from the fact that reproduction by distinct sexes requires the perfection both of the external and internal organs; and if the phenomenon arise, as has just been suggested, from the fact that the internal organs arrive at maturity before the external ones, reproduction will result in those species only which have the power of agamic multiplication.
Moreover, it is evident that we have in the animal kingdom two kinds of dimorphism.
This term has usually been applied to those cases in which animals or plants present themselves at maturity under two forms. Ants and Bees afford us familiar instances among animals; and among plants the interesting case of the genusPrimulahas recently been described by Mr. Darwin. Even more recently he has made known to us the still more remarkable phenomenon afforded by the genusLythrum, in which there are three distinct forms, and which therefore offers an instance of polymorphism.51
The other kind of dimorphism or polymorphism differs from the first in being the result of the differentiating action of external circumstances, not on the mature, but on the young individual. Such different forms, therefore, stand towards one another in the relation of succession. In the first kind the chain of being divides at the extremity; in the other itis composed of dissimilar links. Many instances of this second form of dimorphism have been described under the name of alternation of generations.
The term, however, has met with much opposition, and is clearly inapplicable to the differences exhibited by insects in various periods of their life. Strictly speaking, the phenomena are frequently not alternate, and in the opinion of some eminent naturalists they are not, strictly speaking, cases of generation at all.52
In order, then, to have some name for these remarkable phenomena, and to distinguish them from those cases in which thematureanimal or plant is represented by two or more different forms, I think it would be convenient to retain exclusively for these latter the terms dimorphism and polymorphism; and those cases in which animals or plants pass through a succession of different forms might be distinguished by the name of dieidism or polyeidism.
The conclusions, then, which I think we may draw from the preceding considerations, are:—
1. That the occurrence of metamorphoses arises from the immaturity of the condition in which some animals quit the egg.
2. That the form of the insect larva depends in great measure on the conditions in which it lives. The external forces acting upon it are different from those which affect the mature form; and thus changes are produced in the young, having reference to its immediate wants, rather than to its final form.
3. That metamorphoses may therefore be divided into two kinds, developmental and adaptional or adaptive.
4. That the apparent abruptness of the changes which insects undergo, arises in great measure from the hardness of their skin, which admits of no gradual alteration of form, and which is itself necessary in order to afford sufficient support to the muscles.
5. The immobility of the pupa or chrysalis depends on the rapidity of the changes going on in it.
6. Although the majority of insects go through three well-marked stages after leaving the egg, still a large number arrive at maturity through a greater or smaller number of slight changes.
7. When the external organs arrive at this final form before the organs of reproduction are matured, these changes are known as metamorphoses; when, on the contrary, the organs of reproduction are functionally perfect before the external organs, or when the creature has the power of budding, then the phenomenon is known as alternation of generations.
"Personne," says Carl Vogt, “en Europe au moins, n’ose plus soutenir la Création indépendante et de toutes pièces des espèces,” and though this statement is perhaps not strictly correct, still it is no doubt true, that the Doctrine of Evolution, in some form or other, is accepted by most, if not by all, the greatest naturalists of Europe. Yet it is surprising how much, in spite of all that has been written, Mr. Darwin’s views are still misunderstood. Thus Browning, in one of his recent poems, says:—
"That mass man sprang from was a jelly lumpOnce on a time; he kept an after courseThrough fish and insect, reptile, bird, and beast,Till he attained to be an ape at last,Or last but one."53
"That mass man sprang from was a jelly lumpOnce on a time; he kept an after courseThrough fish and insect, reptile, bird, and beast,Till he attained to be an ape at last,Or last but one."53
This theory, though it would be regarded by many as a fair statement of his views, is one which Mr. Darwin would entirely repudiate. Whether fish and insect, reptile, bird and beast, are derived from one original stock or not, they are certainly not links in onesequence. I do not, however, propose to discuss the question of Natural Selection, but may observe that it is one thing to acknowledge that in Natural Selection, or the survival of the fittest, Mr. Darwin has called attention to avera causa, has pointed out the true explanation of certain phenomena; but it is quite another thing to maintain that all animals are descended from some primordial source.
For my own part, I am satisfied that Natural Selection is a true cause, and, whatever may be the final result of our present inquiries—whether animated nature be derived from one ancestral source, or from many—the publication of the Origin of Species will none the less have constituted an epoch in the History of Biology. But, how far the present condition of living beings is due to that cause; how far, on the other hand, the action of Natural Selection has been modified and checked by other natural laws—by the unalterability of types, by atavism, &c.; how many types of life originally came into being; and whether they arose simultaneously or successively,—these and many other similar questions remain unsolved, even admitting the theory of Natural Selection. All this has indeed been clearly pointed out by Mr. Darwin himself, and would not need repetition but for the careless criticism by which in too many cases the true question has been obscured. Without, however, discussing the argument for and against Mr. Darwin’s conclusions, so often do we meet with travesties of it like that which I have just quoted, that it is well worth while to consider the stages through which some group, say for instance that of insects, have probably come to be what they are, assuming them to have developed under natural laws from simpler organisms. The question is one of great difficulty. It is hardly necessary to say that insects cannot have passed through all the lower forms of animal life, and naturalists do not at present agree as to the actual line of their development.
In the case of insects, the gradual course of evolution through which the present condition of the group has probably been reached, has been discussed by Mr. Darwin, by Fritz Müller, Haeckel, Brauer, myself and others.
In other instances Palæontology throws much light on this question. Leidy has shown that the milk-teeth of the genusEquusresemble the permanent teeth of the ancientAnchitherium, while the milk-teeth ofAnchitheriumagain approximate to the dental system of the still earlierMerychippus. Rütimeyer, while calling attention to this interesting observation, adds that the milk-teeth ofEquus caballusin the same way, and still more those ofE. fossilis, resemble the permanent teeth ofHipparion.
"If we were not acquainted with the horse," says Flower,54“we could scarcely conceive of an animal whose only support was the tip of a single toe on each extremity, to say nothing of the singular conformation of its teeth and other organs. So striking have these characters appeared to many zoologists, that the animals possessing them have been reckoned as an order apart, called Solidungula; but palæontology has revealed that in the structure of its skull, its teeth, its limbs, the horse is nothing more than a modifiedPalæotherium; and though still with gaps in certain places, many of the intermediate stages of these modifications are already known to us, being thePalæotherium,Anchitherium,Merychippus, andHipparion.”
"All Echinoids," says A. Agassiz,55“pass, in their early stages, through a condition which recalls to us the first Echinoids which made their appearance in geological ages.” On embryological grounds, he observes, we should “place true Echini lowest, then the Clypeastroids, next the Echinolamps, and finally the Spatangoids.” Now among the Echinoids of the Trias there are no Clypeastroids, Echinolamps, or Spatangoids. The Clypeastroids make their appearance in the Lias, the Echinolamps in the Jurassic, while the Spatangoids commence in the Cretaceous period.
Again56“in the Radiates, the Acalephs in their first stages of growth, that is, in their Hydroid condition, remind us of the adult forms among Polyps, showing the structural rank of the Acalephs to be the highest, since they pass beyond a stage which is permanent with the Polyps; while the Adult forms of the Acalephs have in their turn a certain resemblance to the embryonic phases of the class next above them, the Echinoderms; within the limits of the classes, the same correspondence exists as between the different orders; the embryonic forms of the highest Polyps recall the adult forms of the lowerones, and the same is true of the Acalephs as far as these phenomena have been followed and compared among them.” Indeed, the accomplished authors from whom I have taken the above quotation, do not hesitate to say57that “whenever such comparisons have been successfully carried out, the result is always the same; the present representatives of the fossil types recall in their embryonic condition the ancient forms, and often explain their true position in the animal kingdom.”
Fossil insects are unfortunately rare, there being but few strata in which the remains of this group are well preserved. Moreover, well-characterized Orthoptera and Neuroptera occur as early as the Devonian strata; Coleoptera and Hemiptera in the Coal-measures; Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the contrary, not until the Tertiary. But although it appears from these facts that, as far as our present information goes, the Orthoptera and Neuroptera are the most ancient orders, it is not, I think, conceivable that the latter should have been derived from any known species of the former; on the other hand, the earliest known Neuroptera and Orthoptera, though in some respects less specialized than existing forms, are as truly, and as well characterized, Insects, as any now existing; nor are we acquainted with any earlier forms, which in any way tend to bridge over the gap between them and lower groups, though, as we shall see, there are types yet existing which throw much light on the subject.
In the consideration then of this question, we must rely principally on Embryology and Development. I have already referred to the cases in which species, very unlike in their mature condition, are very similar one to another when young. Haeckel, in his “Naturliche Schöpfungsgeschichte,” gives a diagram which illustrates this very well as regards Crustacea. Pls. 1-4 show the same to be the case with Insects.
The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat, the Grasshopper,—these and other less familiar types seem at first to have little in common. They differ in size, in form, in colour, in habits, and modes of life. Yet the researches of entomologists, following the clue supplied by the illustrious Savigny, have proved, not only that while differing greatly in details, they are constructed on one common plan; but also that other groups, as for instance, Crustacea (Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown to be fundamentally similar. In Pl.IVI have figured the larvæ of anEphemera(Fig. 1), of aMeloë(Fig. 2), of a Dragon-fly (Fig. 3), of aSitaris(Fig. 4), of aCampodea(Fig. 5), of aDyticus(Fig. 6), of a Termite (Fig. 7), of aStylops(Fig. 8), and of aThrips(Fig. 9). All these larvæ possess many characters in common. The mature forms are represented in the corresponding figures of Plate 3, and it will at once be seen how considerably they differ from one another. The same fact is also illustrated in Figs.48-55, where Figs.48-51represent the larval states of the mature forms represented in Figs.52-55. Fig.48is the larva of a moth,Agrotis suffusa(Fig.52); Fig.49ofa beetle,Haltica(Fig.53); Fig.50of a Saw-fly,Cimbex(Fig.54); and Fig.51of a Centipede,Julus(Fig.55).
Figs. 48-51Fig. 48, Larva of Moth (Agrotis suffusa), after Packard. 49, Larva of Beetle (Haltica), after Westwood. 50, Larva of Sawfly (Cimbex), Brischke and Zaddach. Beob. ub d. arten. der Blatt und Holzwespen, Fig. 8. 51, Larva ofJulus. Newport, Philos. Transactions, 1841.
Fig. 48, Larva of Moth (Agrotis suffusa), after Packard. 49, Larva of Beetle (Haltica), after Westwood. 50, Larva of Sawfly (Cimbex), Brischke and Zaddach. Beob. ub d. arten. der Blatt und Holzwespen, Fig. 8. 51, Larva ofJulus. Newport, Philos. Transactions, 1841.
Thus, then, although it can be demonstrated that perfect insects, however much they differ in appearance, are yet reducible to one type, the fact becomes much more evident if we compare the larvæ. M. Brauer58and I59have pointed out that two types of larvæ, which I have proposed to callCampodea-form andLindia-form, and which Packard has named Leptiform and Eruciform, run through the principal groups of insects. This is obviously a fact of great importance: as all individualMeloës are derived from a form resembling Pl.II, Fig. 2, it is surely no rash hypothesis to suggest that the genus itself may have been so.
Figs. 52-53Fig. 52,Agrotis suffusa(after Packard). 53,Haltica(after Westwood).
Fig. 52,Agrotis suffusa(after Packard). 53,Haltica(after Westwood).
Fig. 54Fig. 54,Cimbex, Brischae and Zaddach. l.c. T. 2, Fig. 9.
Fig. 54,Cimbex, Brischae and Zaddach. l.c. T. 2, Fig. 9.
Fig. 55Fig. 55.Julus(after Gervais).
Fig. 55.Julus(after Gervais).
Firstly, however, let me say a word as to the general Insect type. It may be described shortly as consisting of animals possessing a head, with mouth parts, eyes and antennæ; a many segmented body, with three pairs of legs on the segments immediately following the head; with, when mature, either one or two pairs of wings, generally with caudal appendages I will not now enter into a description of their internal anatomy. It will be seen that, exceptas regards the wings, Pl.IV, Fig. 4, representing the larva of a small beetle namedSitaris, answers very well to this description. Many other Beetles are developed from larvæ closely resembling those ofMeloë(Pl.IV, Fig. 2), andSitaris(Pl.IV, Fig. 4); in fact—except those species the larvæ of which, as, for instance of theWeevils(Pl.II, Fig. 6), are internal feeders, and do not require legs—we may say that the Coleoptera generally are derived from larvæ of this type.
I will now pass to a second order, the Neuroptera. Pl.IV, Fig. 1, represents the larva ofChloëon, a species the metamorphoses of which I described some years ago in the Linnean Transactions,60and it is obvious that in essential points it closely resembles the form to which I have just alluded.
The Orthoptera, again, the order to which Grasshoppers, Crickets, Locusts, &c. belong, commence life in a similar condition; and the same may also be said of the Trichoptera.
The larvæ of Bees when they quit the egg are entirely legless, but in an earlier stage they possess well-marked rudiments of thoracic legs, showing, as it seems to me, that their apodal condition is an adaptation to their circumstances. Other Hymenopterous larvæ, those for example ofSirex(Fig.9), and of the Saw-flies (Fig.50) have well-developed thoracic legs.
From the difference in external form, and especially from the large comparative size of the abdomen, these larvæ, as well as those of Lepidoptera (Fig.48),have generally been classed with the maggots of Flies,Weevils, &c., rather than with the more active form of larva just adverted to. This seems to me, as I have already pointed out,61to be a mistake. The caterpillar type differs, no doubt, in its general appearance, owing to its greater clumsiness, but still essentially agrees with that already described.
No Dipterous larva, so far as I know, belongs truly to this type; in fact, the early stages of the pupa in the Diptera seem in some respects to correspond to the larvæ of other Insect orders. The Development of the Diptera is, however, as Weissman62has shown, very abnormal in other respects.
Thus, then, we find in many of the principal groups of insects that, greatly as they differ from one another in their mature condition, when they leave the egg they more nearly resemble the typical insect type; consisting of a head; a three-segmented thorax, with three pairs of legs; and a many-jointed abdomen, often with anal appendages. Now, is there any mature animal which answers to this description? We need not have been surprised if this type, through which it would appear that insects must have passed so many ages since (for winged Neuroptera have been found in the carboniferous strata) had long ago become extinct. Yet it is not so. The interesting genusCampodea(Pl.III, Fig. 5) still lives; it inhabits damp earth, and closely resembles the larva ofChloëon(Pl.II, Fig. 1), constituting, indeed, a type which, as shown in Pl. 4,occurs in many orders of insects. It is true that the mouth-parts ofCampodeado not resemble either the strongly mandibulate form which prevails among the larvæ of Coleoptera, Orthoptera, Neuroptera, Hymenoptera, Lepidoptera; or the suctorial type of the Homoptera and Heteroptera. It is, however, not the less interesting or significant on that account, since, as I have elsewhere63pointed out, its mouth-parts are intermediate between the mandibulate and haustellate types; a fact which seems to me most suggestive.
It appears, then, that there are good grounds for considering that the various types of insects are descended from ancestors more or less resembling the genusCampodea, with a body divided into head, thorax, and abdomen: the head provided with mouth-parts, eyes, and one pair of antennæ; the thorax with three pairs of legs; and the abdomen, in all probability, with caudal appendages.
If these views are correct, the genusCampodeamust be regarded as a form of remarkable interest since it is the living representative of a primæval type, from which not only the Collembola and Thysanura, but the other great orders of insects have derived their origin.
From what lower group theCampodeatype was itself derived is a question of great difficulty. Fritz Müller indeed says,64“if all the classes of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident thatthe water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial classes, with their tracheal respiration, have branched off.” Haeckel, moreover, is of the opinion that the Tracheata are developed from the Crustacea, and probably from the Zoëpoda. For my own part, though I feel very great diffidence in expressing an opinion at variance with that of such high authorities, I am rather disposed to suggest that theCampodeatype may possibly have been derived from a less highly developed one, resembling the modern Tardigrade,65a (Fig.56) smaller and much less highly organized being thanCampodea. It possesses two eyes, three anterior pairs of legs, and one at the posterior end of the body, giving it a curious resemblance to some Lepidopterous larvæ.
Fig. 56Fig. 56, Tardigrade (after Dujardin).
Fig. 56, Tardigrade (after Dujardin).
These legs, however, as will be seen, are reduced to mere projections. But for them, the Tardigradawould closely resemble the vermiform larva so common among insects. Among Trichoptera the larva early acquires three pairs of legs, but as Zaddach has shown,66there is a stage, though it is quickly passed through, in which the divisions of the body are indicated, but no trace of legs is yet present. Indeed, there appear to be reasons for considering that while among Crustacea the appendages appear before the segments, in Insects the segments precede the appendages, although this stage of development is very transitory, and apparently, in some cases, altogether suppressed. I say “apparently,” because, as I have already mentioned, I am not yet satisfied that it will not eventually be found to be so in all cases. Zaddach, in his careful observations of the embryology ofPhryganea, only once found a specimen in this stage, which also, according to the researches of Huxley,67seems to be little more than indicated inAphis. It is therefore possible that in other cases, when no such stage has been observed, it not really may be absent, but, from its transitoriness, may have hitherto escaped attention.
Fritz Müller has expressed the opinion68that this vermiform type is of comparatively recent origin. He says: “The ancient insects approached more nearly to the existing Orthoptera, and perhaps to the wingless Blattidæ, than to any other order, and the complete metamorphosis of the Beetles, Lepidoptera, &c., is of later origin.” “There were,” he adds, “perfect insectsbefore larvæ and pupæ.” This opinion has been adopted by Mr. Packard69in his “Embryological Studies on Hexapodous Insects.”
M. Brauer70also considers that the vermiform larva is a more recent type than the Hexapod form, and is to be regarded not as a developmental form, but as an adaptational modification of the earlier active hexapod type. In proof of this he quotes the case ofSitaris.
Figs. 57-58Fig. 57, Larva ofCecidomyia(After Packard). 58,Lindia torulosa(after Dujardin).
Fig. 57, Larva ofCecidomyia(After Packard). 58,Lindia torulosa(after Dujardin).
Considering, however, the peculiar habits of this genus, to which I have already referred, and also that the vermiform type is altogether lower in organization and less differentiated than theCampodeaform, I cannot but regard this case as exceptional; one in which the development has been, as it were, to use an expression of Fritz Müller’s, “falsified” by the struggle for existence, and which therefore does not truly indicate the successive stages of evolution. On the whole, the facts seem to me to point to the conclusion that, though the grublike larvæ of Coleoptera and some other insects, owe their present form mainly to the influence of external circumstances, and partially also to atavism, still theCampodeatype is itself derived from earlier vermiform ancestors. Nicolas Wagner has shown in the case of a small gnat, allied toCecidomyia, that even now, in some instances, the vermiform larvæ possess the power of reproduction. Such a larva (as, for instance, Fig.57) very closely resembles some of the Rotatoria, such for instance asAlbertiaorNotommata, which howeverpossess vibratile cilia. There is, indeed, one genus—Lindia(Fig.58)—in which these ciliæ are altogether absent, and which, though resemblingMacrobiotusin many respects, differs from that genus in being entirely destitute of legs. I have never met with it myself, but it is described by Dujardin, who found it in a ditch near Paris, as being oblong, vermiform, divided into rings, and terminating posteriorly in two short conical appendages. The jaws are not unlike those of the larvæ of Flies, and indeed many naturalists meeting with such a creature would, I am sure, regard it as a small Dipterous larva; yetDujardin figures a specimen containing an egg, and seems to have no doubt that it is a mature form.71
For the next descending stage we must, I think, look among the Infusoria, through such genera asChætonotusorIchthydium. Other forms of the Rotatoria, such for instance asRattulus, and still more the very remarkable species discovered in 1871 by Mr. Hudson,72and described under the name ofPedalion mira, seem to lead to the Crustacea through the Nauplius form. Dr. Cobbold tells me that he regards theGordiias the lowest of the Scolecida; Mr. E. Ray Lankester considers some of the Turbellaria, such genera asMesostomum,Vortex, &c., to be the lowest of existing worms; excluding the parasitic groups. Haeckel73also regards the Turbellaria as forming the nearest approach to the Infusoria. The true worms seem, however, to constitute a separate branch of the animal kingdom.
Fig. 59Fig. 59,Prorhynchus stagnaus.75
Fig. 59,Prorhynchus stagnaus.75
We may take, as an illustration of the lower worms, the genusProrhynchus(Fig.59), which consists of a hollow cylindrical body, containing a straight simple tube, the digestive organ.
But however simple such a creature as this may be, there are others which are far less complex, far less differentiated; which therefore, on Mr. Darwin’s principles, may be considered still more closely to represent the primæval ancestor from which these more highly-developed types have been derived, and which, in spite of their great antiquity—in spite of, or perhaps in consequence of, their simplicity, still maintain themselves almost unaltered.
Thus the form which Haeckel has described74under the nameProtamœba primitiva, Pl.V, Fig. 1-5, consists of a homogeneous and structureless substance, which continually alters its form; putting out and drawing in again more or less elongated processes, and creeping about like a trueAmœba, from which, however,Protamœbadiffers, in the absence of a nucleus. It seems difficult to imagine anything simpler; indeed, as described, it appears to be an illustration of properties without structure. It takes into itself any suitable particle with which it comes in contact, absorbs that which is nutritious, and rejects the rest. From time to time a constriction appears at the centre (Pl.V, Fig. 2), its form approximates more and more to that of an hour-glass (Pl.V, Fig. 3), and at length the two halves separate, and each commences an independent existence (Pl.V, Fig. 5).