50 THE EVOLUTION THEOKY 



with wholly or almost wholly unchanged ids may come together in 

 one germ-cell, and if another sperm-cell of this kind meets with g,n 

 egg-cell similarly constituted, an individual of the old species must 

 arise. But this must — on our assumption — be at a disadvantage as 

 compared with the transformed individuals in the struggle for exist- 

 ence, and will perish in it, and therefore the number of unmodified 

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

 obvious, however, that this will take place very slowly, as we may 

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

 speak later on. 



But what is true of the ids is true also of their constituent parts, 

 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 collectively exert 

 a controlling influence on the cells, not even on the germ-cells, whose 

 histological differentiation into spermatic or egg-cells can only depend 

 on control through specific sex-cell determinants. It is the different 

 determinants of the ids that control ; transformations of the species 

 wnll, 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, Lyccena aged Is in 

 Germany and Lycmna artaxerxes in Scotland, only differ from each 

 other in that the black spot in the middle of the wing in L. agestis 

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

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

 spot. In a majority of the ids in L. artaxerxes the determinants of 

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

 ' milk-white.' This majority will increase very slov/l}^ if the white 

 colour has no pronounced advantage for the persistence of the 

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

 extremely slowly, through the elimination of those individuals whose 

 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 

 ncAV 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 either simultaneously 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 equivalent 

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



