Facts and Arguments for Darwin, Fritz Muller [ebook reader computer .TXT] 📗

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 destitute 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. This is a circumstance which renders very doubtful the equivalence of the middle-body of the Malacostraca with the section of the body which in the Copepoda bears the swimming feet and in the Cirripedia the cirri.

The comprehension of the feet of the hind-body and tail in a single group (as “fausses pattes abdominales,” or as “pleopoda”) seems not to be justifiable. When there is a metamorphosis, they are probably always produced at different periods, and they are almost always quite different in structure and function. Even in the Amphipoda, in which the caudal feet usually resemble in appearance the last two pairs of abdominal feet, they are in general distinguished by some sort of peculiarity, and whilst the abdominal feet are reproduced in wearisome uniformity throughout the entire order, the caudal feet are, as is well-known, amongst the most variable parts of the Amphipoda.)

And if at the present day the majority of the Crabs and Macrura, and indeed the Stalk-eyed Crustacea in general, pass through Zoea-like developmental states, and the same mode of transformation was to be ascribed to their ancestors, the same thing must also apply, if not to the immediate ancestors of the Amphipoda and Isopoda, at least to the common progenitors of these and the Stalk-eyed Crustacea. Any such assumption as this was, however, very hazardous, so long as not a single fact properly relating to the Edriophthalma could be adduced in its support, as the structure of this very coherent group seemed to be almost irreconcilable with many peculiarities of the Zoea. Thus, in my eyes, this point long constituted one of the chief difficulties in the application of the Darwinian views to the Crustacea, and I could scarcely venture to hope that I might yet find traces of this passage through the Zoea-form among the Amphipoda or Isopoda, and thus obtain a positive proof of the correctness of this conclusion. At this point Van Beneden’s statement that a cheliferous Isopod (Tanais Dulongii), belonging, according to Milne-Edwards, to the same family as the common Asellus aquaticus, possesses a carapace like the Decapoda, directed my attention to these animals, and a careful examination proved that these Isopods have preserved, more truly than any other adult Crustacea, many of the most essential peculiarities of the Zoeae, especially their mode of respiration. Whilst in all other Oniscoida the abdominal feet serve for respiration, these in our cheliferous Isopod (Figure 2) are solely motory organs, into which no blood-corpuscle ever enters, and the chief seat of respiration is, as in the Zoeae, in the lateral parts of the carapace, which are abundantly traversed by currents of blood, and beneath which a constant stream of water passes, maintained, as in Zoeae and the adult Decapoda, by an appendage of the second pair of maxillae, which is wanting in all other Edriophthalma.

For both these discoveries, it may be remarked in passing, science is indebted less to a happy chance than immediately to Darwin’s theory.

Species of Peneus live in the European seas, as well as here, and their Nauplius-brood has no doubt repeatedly passed unnoticed through the hands of the numerous naturalists who have investigated those seas, as well as through my own,* for it has nothing which could attract particular attention amongst the multifarious and often wonderful Nauplius-forms. (* Mecznikow has recently found Naupliiform shrimp-larvae in the sea near Naples.) When I, fancying from the similarity of its movements that it was a young Peneus-Zoea, had for the first time captured such a larva, and on bringing it under the microscope found a Nauplius differing toto coelo from this Zoea, I might have thrown it aside as being completely foreign to the developmental series which I was tracing, if the idea of early Naupliiform stages of the higher Crustacea, which indeed I did not believe to be still extant, had not at the moment vividly occupied my attention.

And if I had not long been seeking among the Edriophthalma for traces of the supposititious Zoea-state, and seized with avidity upon everything that promised to made this refractory Order serviceable to me, Van Beneden’s short statement could hardly have affected me so much in the manner of an electric shock, and impelled me to a renewed study of the Tanaides, especially as I had once before plagued myself with them in the Baltic, without getting any further than my predecessors, and I have not much taste for going twice over the same ground.

 

CHAPTER 4. SEXUAL PECULIARITIES AND DIMORPHISM.

Our Tanais, which in nearly all the particulars of its structure is an extremely remarkable animal, furnished me with a second fact worthy of notice in connection with the theory of the origin of species by natural selection.

When hand-like or cheliform structures occur in the Crustacea, these are usually more strongly developed in the males than in the females, often becoming enlarged in the former to quite a disproportionate size, as we have already seen to be the case in Melita. A better known example of such gigantic chelae is presented by the males of the Calling Crabs (Gelasimus), which are said in running to carry these claws “elevated, as if beckoning with them”—a statement which, however, is not true of all the species, as a small and particularly large-clawed one, which I have seen running about by thousands in the cassava-fields at the mouth of the Cambriu, always holds them closely pressed against its body.

A second peculiarity of the male Crustacea consists not unfrequently in a more abundant development on the flagellum of the anterior antennae of delicate filaments which Spence Bate calls “auditory cilia,” and which I have considered to be olfactory organs, as did Leydig before me, although I was not aware of it. Thus they form long dense tufts in the males of many Diastylidae, as Van Beneden also states with regard to Bodotria, whilst the females only possess them more sparingly. In the Copepoda, Claus called attention to the difference of the sexes in this respect. It seems to me, as I may remark in passing, that this stronger development in the males is greatly in favour of the opinion maintained by Leydig and myself, as in other cases male animals are not unfrequently guided by the scent in their pursuit of the ardent females.

Now, in our Tanais, the young males up to the last change of skin preceding sexual maturity resemble the females, but then they undergo an important metamorphosis. Amongst other things they lose the moveable appendages of the mouth even to those which serve for the maintenance of the respiratory current; their intestine is always found empty, and they appear only to live for love. But what is most remarkable is, that they now appear under two different forms. Some (Figure 3) acquire powerful, long-fingered, and very mobile chelae, and, instead of the single olfactory filament of the female, have from 12 to 17 of these organs, which stand two or three together on each joint of the flagellum. The others (Figure 5) retain the short thick form of the chelae of the females; but, on the other hand, their antennae (Figure 6) are equipped with a far greater number of olfactory filaments, which stand in groups of from five to seven together.

(FIGURE 3. Head of the ordinary form of the male of Tanais dubius (?) Kr. magnified 90 times. The terminal setae of the second pair of antennae project between the cheliferous feet.

FIGURE 4. Buccal region of the same from below; lambda, labrum.

FIGURE 5. Head of the rarer form of the male, magnified 25 times.

FIGURE 6. Flagellum of the same, with olfactory filaments, magnified 90 times.)

In the first place, and before inquiring into its significance, I will say a word upon this fact itself. It was natural to consider whether two different species with very similar females and very different males might not perhaps live together, or whether the males, instead of occurring in two sharply defined forms, might not be only variable within very wide limits. I can admit neither of these suppositions. Our Tanais lives among densely interwoven Confervae, which form a coat of about an inch in thickness upon stones in the neighbourhood of the shore. If a handful of this green felt is put into a large glass with clear sea-water, the walls of the glass are soon seen covered with hundreds, nay with thousands, of these little, plump, whitish Isopods. In this way I have examined thousands of them with the simple lens, and I have also examined many hundreds with the microscope, without finding any differences among the females, or any intermediate forms between the two kinds of males.

To the old school this occurrence of two kinds of males will appear to be merely a matter of curiosity. To those who regard the “plan of creation” as the “free conception of an Almighty intellect, matured in the thoughts of the latter before it is manifested in palpable, external forms,” it will appear to be a mere caprice of the Creator, as it is inexplicable either from the point of view of practical adaptation, or from the “typical plan of structure.” From the side of Darwin’s theory, on the contrary, this fact acquires meaning and significance, and it appears in return to be fitted to throw light upon a question in which Bronn saw “the first and most material objection against the new theory,” namely, how it is possible that from the accumulation in various directions of the smallest variations running out of one another, varieties and species are produced, which stand out from the primary form clearly and sharply like the petiolated leaf of a Dicotyledon, and are not amalgamated with the primary form and with each other like the irregular curled lobes of a foliaceous Lichen.

Let us suppose that the males of our Tanais, hitherto identical in structure, begin to vary, in all directions as Bronn thinks, for aught I care. If the species was adapted to its conditions of existence, if the BEST in this respect had been attained and secured by natural selection, fresh variations affecting the species as a species would be retrogressions, and thus could have no prospect of prevailing.