Facts and Arguments for Darwin | Page 7

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