Facts and Arguments for Darwin Part 1

Facts and Arguments for Darwin.

by Fritz Muller.

PREFACE.

My princ.i.p.al reason for undertaking the translation of Dr. Fritz Muller's admirable work on the Crustacea, ent.i.tled 'Fur Darwin,' was that it was still, although published as long ago as 1864, and highly esteemed by the author's scientific countrymen, absolutely unknown to a great number of English naturalists, including some who have occupied themselves more or less specially with the subjects of which it treats.

It possesses a value quite independent of its reference to Darwinism, due to the number of highly interesting and important facts in the natural history and particularly the developmental history of the Crustacea, which its distinguished author, himself an unwearied and original investigator of these matters, has brought together in it. To a considerable section of English naturalists the tone adopted by the author in speaking of one of the greatest of their number will be a source of much gratification.

In granting his permission for the translation of his little book, Dr.

Fritz Muller kindly offered to send some emendations and additions to certain parts of it. His notes included many corrections of printers'

errors, some of which would have proved unintelligible without his aid, some small additions and notes which have been inserted in their proper places, and two longer pieces, one forming a footnote near the close of Chapter 11, the other at the end of Chapter 12, describing the probable mode of evolution of the Rhizocephala from the Cirripedia.

Of the execution of the translation I will say but little. My chief object in this, as in other cases, has been to furnish, as nearly as possible, a literal version of the original, regarding mere elegance of expression as of secondary importance in a scientific work. As much of Dr. Muller's German does not submit itself to such treatment very readily, I must beg his and the reader's indulgence for any imperfections arising from this cause.

W.S.D.

LONDON, 15TH FEBRUARY, 1869.

AUTHOR'S PREFACE.

It is not the purpose of the following pages to discuss once more the arguments deduced for and against Darwin's theory of the origin of species, or to weigh them one against the other. Their object is simply to indicate a few facts favourable to this theory, collected upon the same South American ground, on which, as Darwin tells us, the idea first occurred to him of devoting his attention to "the origin of species,--that mystery of mysteries."

It is only by the acc.u.mulation of new and valuable material that the controversy will gradually be brought into a state fit for final decision, and this appears to be for the present of more importance than a repeated a.n.a.lysis of what is already before us. Moreover, it is but fair to leave it to Darwin himself at first to beat off the attacks of his opponents from the splendid structure which he has raised with such a master-hand.

F.M.

DESTERRO, 7TH SEPTEMBER, 1863.

HISTORY OF CRUSTACEA.

CHAPTER 1. INTRODUCTORY.

When I had read Charles Darwin's book 'On the Origin of Species,' it seemed to me that there was one mode, and that perhaps the most certain, of testing the correctness of the views developed in it, namely, to attempt apply them as specially as possible to some particular group of animals. such an attempt to establish a genealogical tree, whether for the families of a cla.s.s, the genera of a large family, or for the species of an extensive genus, and to produce pictures as complete and intelligible as possible of the common ancestors of the various smaller and larger circles, might furnish a result in three different ways.

1. In the first place, Darwin's suppositions when thus applied might lead to irreconcilable and contradictory conclusions, from which the erroneousness of the suppositions might be inferred. If Darwin's opinions are false, it was to be expected that contradictions would accompany their detailed application at every step, and that these, by their c.u.mulative force, would entirely destroy the suppositions from which they proceeded, even though the deductions derived from each particular case might possess little of the unconditional nature of mathematical proof.

2. Secondly, the attempt might be successful to a greater or less extent. If it was possible upon the foundation and with the aid of the Darwinian theory, to show in what sequence the various smaller and larger circles had separated from the common fundamental form and from each other, in what sequence they had acquired the peculiarities which now characterise them, and what transformations they had undergone in the lapse of ages,--if the establishment of such a genealogical tree, of a primitive history of the group under consideration, free from internal contradictions, was possible,--then this conception, the more completely it took up all the species within itself, and the more deeply it enabled us to descend into the details of their structure, must in the same proportion bear in itself the warrant of its truth, and the more convincingly prove that the foundation upon which it is built is no loose sand, and that it is more than merely "an intellectual dream."

3. In the third place, however, it was possible, and this could not but appear, prima facie, the most probable case, that the attempt might be frustrated by the difficulties standing in its way, without settling the question, either way, in a perfectly satisfactory manner. But if it were only possible in this way to arrive for oneself at a moderately certain independent judgment upon a matter affecting the highest questions so deeply, even this alone could not but be esteemed a great gain.

Having determined to make the attempt, I had in the first place to decide upon some particular cla.s.s. The choice was necessarily limited to those the chief forms of which were easily to be obtained alive in some abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the Diastylidae, the Amphipoda and Isopoda, the Ostracoda and Daphnidae, the Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast, representing the cla.s.s of Crustacea with the deficiency only of the Phyllopoda and Xiphosura, furnished a long and varied, and at the same time intimately connected series, such as was at my command in no other cla.s.s. But even independently of this circ.u.mstance the selection of the Crustacea could hardly have been doubtful. Nowhere else, as has already been indicated by various writers, is the temptation stronger to give to the expressions "relationship, production from a common fundamental form," and the like, more than a mere figurative signification, than in the case of the lower Crustacea. Among the parasitic Crustacea, especially, everybody has long been accustomed to speak, in a manner scarcely admitting of a figurative meaning, of their arrest of development by parasitism, as if the transformation of species were a matter of course. It would certainly never appear to any one to be a pastime worthy of the Deity, to amuse himself with the contrivance of these marvellous cripplings, and so they were supposed to have fallen by their own fault, like Adam, from their previous state of perfection.

That a great part of the larger and smaller groups into which this cla.s.s is divided, might be regarded as satisfactorily established, was a further advantage not to be undervalued; whilst in two other cla.s.ses with which I was familiar, namely, the Annelida and Acalephae, all the attempted arrangements could only be considered preliminary revisions.

These undisplaceable groups, like the sharply marked forms of the hard, many-jointed dermal framework, were not only important as safe starting points and supports, but were also of the highest value as inflexible barriers in a problem in which, from its very nature, fancy must freely unfold her wings.

When I thus began to study our Crustacea more closely from this new stand-point of the Darwinian theory,--when I attempted to bring their arrangements into the form of a geological tree, and to form some idea of the probable structure of their ancestors,--I speedily saw (as indeed I expected) that it would require years of preliminary work before the essential problem could be seriously handled. The extant systematic works generally laid more weight upon the characters separating the genera, families and orders, than upon those which unite the members of each group, and consequently often furnished but little employable material. But above all things a thorough knowledge of development was indispensable, and every one knows how imperfect is our present knowledge of this subject. The existing deficiencies were the more difficult to supply, because, as Van Beneden remarks with regard to the Decapoda, from the often incredible difference in the development of the most nearly allied forms, these must be separately studied--usually family by family, and frequently genus by genus--nay, sometimes, as in the case of Peneus, even species by species; and because these investigations, in themselves troublesome and tedious, often depend for their success upon a lucky chance.

But although the satisfactory completion of the "Genealogical tree of the Crustacea" appeared to be an undertaking for which the strength and life of an individual would hardly suffice, even under more favourable circ.u.mstances than could be presented by a distant island, far removed from the great market of scientific life, far from libraries and museums--nevertheless its practicability became daily less doubtful in my eyes, and fresh observations daily made me more favourably inclined towards the Darwinian theory.

In determining to state the arguments which I derived from the consideration of our Crustacea in favour of Darwin's views, and which (together with more general considerations and observations in other departments), essentially aided in making the correctness of those views seem more and more palpable to me, I am chiefly influenced by an expression of Darwin's: "Whoever," says he ('Origin of Species' page 482), "is led to believe that species are mutable, will do a good service by conscientiously expressing his conviction." To the desire expressed in these words I respond, for my own part, with the more pleasure, as this furnishes me with an opportunity of publicly giving expression in words to the thanks which I feel most deeply to be due from me to Darwin for the instructions and suggestions for which I am so deeply indebted to his book. Accordingly I throw this sand-grain with confidence into the scale against "the load of prejudice by which this subject is overwhelmed," without troubling myself as to whether the priests of orthodox science will reckon me amongst dreamers and children in knowledge of the laws of nature.

CHAPTER 2. THE SPECIES OF MELITA.

A false supposition, when the consequences proceeding from it are followed further and further, will sooner or later lead to absurdities and palpable contradictions. During the period of tormenting doubt--and this was by no means a short one--when the pointer of the scales oscillated before me in perfect uncertainty between the pro and the con, and when any fact leading to a quick decision would have been most welcome to me, I took no small pains to detect some such contradictions among the inferences as to the cla.s.s of Crustacea furnished by the Darwinian theory. But I found none, either then, or subsequently. Those which I thought I had found were dispelled on closer consideration, or actually became converted into supports for Darwin's theory.

Nor, so far as I am aware, have any of the NECESSARY consequences of Darwin's hypotheses been proved by any one else, to stand in clear and irreconcilable contradiction. And yet, as the most profound students of the animal kingdom are amongst Darwin's opponents, it would seem that it ought to have been an easy matter for them to crush him long since beneath a ma.s.s of absurd and contradictory inferences, if any such were to be drawn from his theory. To this want of demonstrated contradictions I think we may ascribe just the same importance in Darwin's favour, that his opponents have attributed to the absence of demonstrated intermediate forms between the species of the various strata of the earth. Independently of the reasons which Darwin gives for the preservation of such intermediate forms being only exceptional, this last mentioned circ.u.mstance will not be regarded as of very great significance by any one who has traced the development of an animal upon larvae fished from the sea, and had to seek in vain for months, and even years, for those transitional forms, which he nevertheless knew to be swarming around him in thousands.

A few examples may show how contradictions might come forth as necessary results of the Darwinian hypotheses.

It seems to be a necessity for all crabs which remain for a long time out of the water (but why is of no consequence to us here), that air shall penetrate from behind into the branchial cavity. Now these crabs, which have become more or less estranged from the water, belong to the most different families--the Raninidae (Ranina), Eriphinae (Eriphia gonagra), Grapsoidae (Aratus, Sesarma, etc.), Ocypodidae (Gelasimus, Ocypoda), etc., and the separation of these families must doubtless be referred to a much earlier period than the habit of leaving the water displayed by some of their members. The arrangements connected with aerial respiration, therefore, could not be inherited from a common ancestor, and could scarcely be accordant in their construction. If there were any such accordance not referable to accidental resemblance among them, it would have to be laid in the scale as evidence against the correctness of Darwin's views. I shall show hereafter how in this case the result, far from presenting such contradictions, was rather in the most complete harmony with what might be predicted from Darwin's theory.

(FIGURE 1. Melita exilii n. sp., male, enlarged five times. The large branchial lamellae are seen projecting between the legs.)

A second example.--We are already acquainted with four species of Melita (M. valida, setipes, anisochir, and Fresnelii), and I can add a fifth (Figure 1), in which the second pair of feet bears upon one side a small hand of the usual structure, and on the other an enormous clasp-forceps.

This want of symmetry is something so unusual among the Amphipoda, and the structure of the clasp-forceps differs so much from what is seen elsewhere in this order, and agrees so closely in the five species, that one must unhesitatingly regard them as having sprung from common ancestors belonging to them alone among known species. But one of these species, M. Fresnelii, discovered by Savigny, in Egypt, is said to want the secondary flagellum of the anterior antennae, which occurs in the others. From the trustworthiness of all Savigny's works there can scarcely be a doubt as to the correctness of this statement. Now, if the presence or absence of the secondary flagellum possessed the significance of a distinctive generic character, which is usually ascribed to it, or if there were other important differences between Melita Fresnelii and the other species above-mentioned, which would make it seem natural to separate M. Fresnelii as a distinct genus, and to leave the others united with the rest of the species of Melita--that is to say, in the sense of the Darwinian theory, if we a.s.sume that all the other Melitae possessed common ancestors, which were not at the same time the ancestors of M. Fresnelii--this would stand in contradiction to the conclusion, derived from the structure of the clasp-forceps, that M.

Fresnelii and the four other species above-mentioned possessed common ancestors, which were not also the ancestors of the remaining species of Melita. It would follow:--

1. From the structure of the clasp-forceps: that M. exilii, etc. and M.

Fresnelii would branch off together from a stem which branches off from M. palmata.

2. From the presence or absence of the secondary flagellum: that M.

palmata, etc. and M. exilii, etc. would branch off together from a stem which branches off from M. Fresnelii.

As, in the first case, among the Crabs, a typical agreement of arrangements produced independently of each other would have been a very suspicious circ.u.mstance for Darwin's theory, so also, in the second, would any difference more profound than that of very nearly allied species. Now it seems to me that the secondary flagellum can by no means furnish a reason for doubting the close relationship of M. Fresnelii to M. exilii, etc., which is indicated by the peculiar structure of the unpaired clasp-forceps. In the first place we must consider the possibility that the secondary flagellum, which is not always easy to detect, may only have been overlooked by Savigny, as indeed Spence Bate supposes to have been the case. If it is really deficient it must be remarked that I have found it in species of the genera Leucothoe, Cyrtophium and Amphilochus, in which genera it was missed by Savigny, Dana and Spence Bate--that a species proved by the form of the Epimera (c.o.xae Sp. B.) of the caudal feet (uropoda Westw.), etc., to be a true Amphithoe* possesses it (* I accept this and all the other genera of Amphipoda here mentioned, with the limits given to them by Spence Bate ('Catalogue of Amphipodous Crustacea').)--that in many species of Cerapus it is reduced to a scarcely perceptible rudiment--nay, that it is sometimes present in youth and disappears (although perhaps not without leaving some trace) at maturity, as was found by Spence Bate to be the case in Acanthonotus Owenii and Atylus carinatus, and I can affirm with regard to an Atylus of these seas, remarkable for its plumose branchiae--and that from all this, at the present day when the increasing number of known Amphipoda and the splitting of them into numerous genera thereby induced, compels us to descend to very minute distinctive characters, we must nevertheless hesitate before employing the secondary flagellum as a generic character. The case of Melita Fresnelii therefore cannot excite any doubts as to Darwin's theory.

CHAPTER 3. MORPHOLOGY OF CRUSTACEA--NAUPLIUS-LARVAE.

If the absence of contradictions among the inferences deduced from them for a narrow and consequently easily surveyed department must prepossess us in favour of Darwin's views, it must be welcomed as a positive triumph of his theory if far-reaching conclusions founded upon it should SUBSEQUENTLY be confirmed by facts, the existence of which science, in its previous state, by no means allowed us to suspect. From many results of this kind upon which I could report, I select as examples, two, which were of particular importance to me, and relate to discoveries the great significance of which in the morphology and cla.s.sification of the Crustacea will not be denied even by the opponents of Darwin.

Considerations upon the developmental history of the Crustacea had led me to the conclusion that, if the higher and lower Crustacea were at all derivable from common progenitors, the former also must once have pa.s.sed through Nauplius-like conditions. Soon afterwards I discovered Naupliiform larvae of Shrimps ('Archiv fur Naturgeschichte' 1860 1 page 8), and I must admit that this discovery gave me the first decided turn in Darwin's favour.

(FIGURE 2. Tanais dubius (?) Kr. female, magnified 25 times, showing the orifice of entrance (x) into the cavity overarched by the carapace, in which an appendage of the second pair of maxillae (f) plays. On four feet (i, k, l, m) are the rudiments of the lamellae which subsequently form the brood-cavity.)

The similar number of segments* occurring in the Crabs and Macrura, Amphipoda and Isopoda, in which the last seven segments are always different from the preceding ones in the appendages with which they are furnished, could only be regarded as an inheritance from the same ancestors.

(* Like Claus I do not regard the eyes of the Crustacea as limbs, and therefore admit no ocular segment; on the other hand I count in the median piece of the tail, to which the character of a segment is often denied. In opposition to its interpretation as a segment of the body, only the want of limbs can be cited; in its favour we have the relation of the intestine, which usually opens in this piece, and sometimes even traverses its whole length, as in Microdeutopus and some other Amphipoda. In Microdeutopus, as Spence Bate has already pointed out, one is even led to regard small processes of this tubular caudal piece as rudimentary members. Bell also ('British Stalk-eyed Crustacea' page 20), states that he observed limbs of the last segment in Palaemon serratus in the form of small moveable points.

The attempt has often been made to divide the body of the higher Crustacea into small sections composed of equal numbers of segments, these sections consisting of 3, 5 or 7 segments. None of these attempts has ever met with general acceptance; my own investigations lead me to a conception which nearly approaches Van Beneden's. I a.s.sume four sections of 5 segments each--the primitive body, the fore-body, the hind-body, and the middle-body. The primitive body includes the segments which the naupliiform larva brings with it out of the egg; it is afterwards divided, by the younger sections which become developed in its middle, into the head and tail. To this primitive body belong the two pairs of antennae, the mandibles and the caudal feet ("posterior pair of pleopoda," Sp. B.). Even in the mature animal the fact that these terminal sections belong to one another is sometimes betrayed by the resemblance of their appendages, especially that of the outer branch of the caudal feet, with the outer branch (the so-called scale) of the second pair of antennae. Like the antennae, the caudal feet may also become the bearers of high sensorial apparatus, as is shown by the ear of Mysis.

The sequence of the sections of the body in order of time seems originally to have been, that first the fore-body, then the hind-body, and finally the middle-body was formed. The fore-body appears, in the adult animal, to be entirely or partially amalgamated with the head; its appendages (siagonopoda Westw.) are all or in part serviceable for the reception of food, and generally sharply distinguished from those of the following group. The segments of the middle-body seem always to put forth limbs immediately after their own appearance, whilst the segments of the hind-body often remain dest.i.tute of feet through long portions of the larval life or even throughout life (as in many female Diastylidae), a reason, among many others, for not, as is usual, regarding the middle-body of the Crustacea as equivalent to the constantly footless abdomen of Insects. The appendages of the middle-body (pereiopoda) seem never, even in their youngest form, to possess two equal branches, a peculiarity which usually characterises the appendages of the hind-body.