On the other hand, a transformation in a favourable direction, an adaptation to new conditions of life, as far at least as that implies the simultaneous variation and harmonious co-adaptation of many parts, cannot, as far as I can see, be effected in the course of purely parthenogenetic reproduction, nor can a degeneration of complicated parts which have become superfluous. For both these changes, in my opinion, require that the ids of the germ-plasm should be frequently mingled afresh, since apart from this there cannot be a harmonious readjustment of complicated structures, nor can a uniform degeneration affecting all parts set in. As an example of this last case we may take that organ which became functionless in the purely parthenogenetic species of Ostracods when amphigonous reproduction was given up--the sperm-pocket or receptaculum of the female. All these species still possess an unaltered receptaculum seminis, a large pear-shaped bladder with a long, narrow, spirally coiled entrance-duct, very well adapted for allowing the enormous spermatozoa of the males to make their way in singly, and to arrange themselves within the receptacle side by side in the most beautiful order, like a long ribbon, and finally to migrate out again singly to fertilize the liberated ova. In _Cypris reptans_ and several other species, however, no males have been found in any of the places which have been carefully searched, and the receptaculum of the female is always found to be empty. Nevertheless it shows no hint of degeneration. It is possible enough that, as in _Apus cancriformis_, which is of similar habit, the males have become extinct in most colonies of these species, but that nevertheless they do occur here and there from time to time in the area inhabited by the species, and if this should prove to be the case, it would confirm the conclusion, which is very probable on other grounds, that the pure parthenogenesis of these species has not existed in most of their habitats for a long time, speaking phylogenetically. For this reason we must not over-estimate the significance of the complete persistence of the receptaculum even with exclusively parthenogenetic reproduction. It proves, however, that degeneration of a superfluous organ does not necessarily set in even after hundreds of generations, and in this fact there is certainly a corroboration of the view that it is 'chance' germinal variations which give the impulse to degeneration. These first induce a downgrade variation through germinal selection, and this, if it concerns an organ of no importance to the survival of the species, is not hindered in its progress by personal selection. Whether degeneration of the receptaculum would have occurred in these parthenogenetic species if they had retained even a periodic sexual reproduction, as is actually the case in the generations of the alternately parthenogenetic and sexually reproducing Aphides,