VARIATIONS FAVOURED BY AMPHIMIXIS 159 



The result of the reducing division, as far as its significance 

 for the theory of descent is concerned, is to render the germ- 

 cells of the individual of difierent hereditary value. If we take 

 four chromosomes as the normal number for a species, and 

 suppose two, A and B, to have been inherited by an individual 

 from the mother, and two, C and D, from the father, it is evident 

 that, through the reduction division, either the id combiuation 

 A and B can be separated from C and D, or else A and C from 

 B a,nd D, or else A and D from B and C. Thus, six difEerent 

 id combinations can be realised ia any one germ-cell ; or, in other 

 words, there may be six kinds of germ-cells, each possessing a 

 difEerent id combiuation. If we suppose this " neotaxis " of 

 the germ-plasm to take place in both male and female germ- 

 cells, we shall find that, after fertilisation, 6 x 6=36 hetero- 

 geneous individuals may be produced from the germ-plasm of 

 the two parents. Obviously, the greater the number of chromo- 

 somes, the greater the possible number of combinations. If 

 we take eight as the normal number of chromosomes we shall 

 have seventy possible combiuations, if we take sixteen we have 

 12,870. The possibility of new combinations is immense if we 

 suppose amphimixis between two individuals of such a species, 

 bringing together 70 x 70, or 12,870 x 12,870 potentialities. 

 The normal ntimber of chromosomes in the human species is, 

 as a matter of fact, sixteen, so that the latter figures are applicable 

 in this case. 



We see that the possibility of efEecting new combinations by 

 amphimixis is enormous ; and the chief role of amphimixis is 

 undoubtedly the preservation and multipHcation of variations 

 within a species. For only in so far as a species presents an 

 immense number of variations can natural selection act. 

 Natural selection requires for its action great numbers ; and if a 

 species is to be maintained by the constant adaptation of its 

 members to environing conditions, that species must present 

 a great number of individuals to be selected. If, therefore, the 



