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.