Facts and Arguments for Darwin

by Fritz Müller

Figs. 8 and 9. The two forms of the chelæ of the male of Orchestia Darwinii, magnified.

I cannot refrain from taking this opportunity of remarking that (so far as appears from Spence Bate’s catalogue), for two different kinds of males ( Orchestia telluris and sylvicola ) which live together in the forests of New Zealand, only one form of female is known, and hazarding the supposition that we have here a similar case. It does not seem to me to be probable that two nearly allied species of these social Amphipoda should occur mixed together under the same conditions of life.

Fig. 10. Coxal lamella of the penultimate pair of feet of the male ( a ), and coxal lamella, with the three following joints of the same pair of feet of the female ( b ) of Melita Messalina, magnified.
Fig. 11. Coxal lamella of the same pair of feet of the female of M. insatiabilis.

As the males of several species of Melita are distinguished by the powerful unpaired clasp-forceps, the females of some other species of the same genus are equally distinguished from all other Amphipoda by the circumstance that in them a peculiar apparatus is developed which facilitates their being held by the male. The coxal lamellæ of the penultimate pair of feet are produced into hook-like processes, of which the male lays hold with the hands of the first pair of feet. The two species in which I am acquainted with this structure are amongst the most salacious animals of their order, even females which are laden with eggs in all stages of development, not unfrequently have their males upon their backs. The two species are nearly allied to Melita palmata Leach ( Gammarus Dugesii, Edw.), which is widely distributed on the European coasts, and has been frequently investigated; unfortunately, however, I can find no information as to whether the females of this or any other European species possess a similar contrivance. In M. exilii all the coxal lamellæ are of the ordinary formation. Nevertheless, be this as it will, whether they exist in two or in twenty species, the occurrence of these peculiar hook-like processes is certainly very limited.

Now our two species live sheltered beneath slightly tilted stones in the neighbourhood of the shore: one of them, Melita Messalina, so high that it is but rarely covered by the water; the other, Melita insatiabilis, a little lower; both species live together in numerous swarms. We cannot therefore suppose that the loving couples are threatened with disturbance more frequently than those of other species, nor would it be more difficult for the male, than for those of other species, in case of his losing his female, to find a new one. Nor is it any more easy to see how the contrivance on the body of the female for insuring the act of copulation could be injurious to other species. But so long as it is not demonstrated that our species are particularly in want of this contrivance, or that the latter would rather be injurious than beneficial to other species, its presence only in these few Amphipoda will have to be regarded not as the work of far-seeing wisdom, but as that of a favourable chance made use of by Natural Selection. Under the latter supposition its isolated occurrence is intelligible, whilst we cannot perceive why the Creator blessed just these few species with an apparatus which he found to be quite compatible with the “general plan of structure” of the Amphipoda, and yet denied it to others which live under the same external conditions, and equal them even in their extraordinary salacity. Associated with, or in the immediate vicinity of the two species of Melita, live two species of Allorchestes, the pairs of which are met with almost more numerously than the single animals, and yet their females show no trace of the above-mentioned processes of the coxal lamellæ.

These cases, I think, must be brought to bear against the conception supported with so much genius and knowledge by Agassiz, that species are embodied thoughts of the Creator; and, with these, all similar instances in which arrangements which would be equally beneficial to all the species of a group are wanting in the majority and only conferred upon a few special favourites, which do not seem to want them any more than the rest.

CHAPTER V.
RESPIRATION IN LAND CRABS.

Among the numerous facts in the natural history of the Crustacea upon which a new and clear light is thrown by Darwin’s theory, besides the two forms of the males in our Tanais and in Orchestia Darwinii, there is one which appears to me of particular importance, namely, the character of the branchial cavity in the air-breathing Crabs, of which, unfortunately, I have been unable to investigate some of the most remarkable (Gecarcinus, Ranina). As this character, namely, the existence of an entrance behind the branchiæ, has hitherto been noticed, even as a fact, only in Ranina, I will go into it in some detail. I have already mentioned that, as indeed is required by Darwin’s theory, this entrant orifice is produced in different manners in the different families.