50 THE EVOLUTION THEORY 



with wholly or almost wholly unchanged ids may come to<;other in 

 one germ-cell, and it' another sperm-cell of this kind moots \\ith an 

 egg-cell similarly constituted, an individual of the old spocies nmst 

 arise. Bvit this must— on oiir assumption — be at a disadvantage as 

 compared with the transformed individuals in the struggle for exist- 

 ence, and will perish in it, and thoroforo the number of unmoditlod 

 ids in the germ-plasm of the species will graduallj'- diminish. It is 

 obvious, howevor, that this will take place very slowly, as wo may 

 conclude from the phenomena of reversion, of wJiich I shall have to 

 speak later on. 



But what is true of the ids is trv;e also of their constituent ]iarts, 

 the determinants, and that — if I mistake not — is fundamental in the 

 interpretation of the alternation of hereditary' succession in the parts 

 of the child. 



According to our theory, the ids do not colloctivoly exert 

 a controlling influence on the cells, not oven on the gonu-colls. whose 

 histological diflerentiation into spermatic or egg-cells can only tlopend 

 on control through specific sex- cell determinants. It is the ditt'oront 

 determinants of the ids that control ; transformations of the species 

 will, it is true, depend on transformations of the ids, but this need 

 not necessarily consist in a variation of all the determinants of the 

 id. If, for instance, two species of butterfly, Lycaina cuirdis in 

 (lermany and Lynvna (irtii.riT.ceti in Scotland, only differ from each 

 other in that the black spot in the middle of the wing in L. ((f/rdis 

 is milk-white in L. artaxerxes, no other dotorminnnts in the id of the 

 germ-plasm can be diftoront except those which control this particular 

 spot. In a majority of the ids in L. avtaxevxes the tloterminants of 

 this spot must have boon modified, let us say, to the production of 

 ' milk-white.' This majority will increase very slowly if the white 

 colour has no pronounced advantage for the persistenet' of the 

 species, but it will increase gradually, as we have already seen, though 

 extremely slowly, through the t'limination of those individuals ^^•hose 

 germ-plasm at the reducing division has chanced to receive a majority 

 of ids with the old, unmodified determinants, and which have there- 

 fore reverted to the ancestral form. This will happen whenever the 

 new character has any use, however small, in maintaining the species. 



But in most modifications of species quite a number of parts and 

 characters have undergone variation cuther simultaneoiisly or in rapid 

 succession; in many cases nearly all the details of structure, and 

 therefore almost all the determinants of the germ-plasm, must have 

 varied. We must not, however, assume that all the t-quivalent 

 determinants, for instance, all the determinants K in all the ids, 



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