234 THE EVOLUTION THEORY 



conditions, as has probably always been the case in the experiments 

 in inbreeding. 



It is true that even in old species, living in a state of nature, 

 unfavourable variations may arise in the germ-plasm, and may go on 

 increasing during purely parthenogenetic multiplication, for the ids 

 with unfavourably varying determinants will no longer be set aside 

 by means of reducing division. But those individuals in which the 

 unfavourable variational tendency increases until it has attained 

 selection- value will be subject to selection and will be gradually 

 eliminated ; indeed, the weeding out of the inferior individuals will 

 be more drastic here than where amphigony obtains, because in this 

 case all the offspring of one mother are nearly alike, so that the whole 

 progeny is exterminated if the mother varies unfavourably. 



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 

 pai'ts 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 degene- 

 ration 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 



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