Facts and Arguments for Darwin | Page 4

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 assume 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 circumstance 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 (Coxae 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,