Amphimixis secures adaptation 105 



parts of the human organism. A considerable number of organs 

 and parts of organs are in steady regression — for instance, the 

 eleventh and twelfth ribs, the wisdom teeth, the appendix vermi- 

 formis of the intestine — and although it is ob\dously dif&cult to 

 affirm in any concrete case that similar variation is taking place 

 among the active parts of the organism, there is no reason why 

 this should not be the case. It is believed by many that the 

 increasing histological difierentiation of the cortical zone of the 

 human brain is among the " active progressive variations " 

 which the species is undergoing at the present time. 



And such cases of regressive variation in passive organs, and of 

 progressive variation in active ones, are plainly cases of adapta- 

 tion ; and it is equally clear that regressive variation of useless — 

 consequently passive — organs coidd not take place simultane- 

 ously throughout the whole human species, as it does, were it not 

 that amphimixis " spreads far and wide " the minus variations of 

 these organs, the retention of whose determinants in the germ- 

 plasm would, as we have said in the chapter on Germinal Selec- 

 tion, be fraught with danger to the species. The most constant 

 species never remain wholly unaltered. Change and movement are 

 the laws of organic life, and everywhere we find species involved 

 in a process of perpetual Becoming — throwing off that part of 

 the ancestral legacy which has become cumbersome under altered 

 conditions, perfecting the most useful organs and parts towards 

 fitter adaptation and greater utility, improving the co-ordination 

 of the various parts. During this period of increasing adaptation 

 amphimixis is indispensable, for it is ever disseminating the plus 

 variations of those parts which require to be increased. This 

 indispensable result of amphimixis is sufficient to ensure that 

 natural selection wOl retain it, even if it were not sufficiently 

 guaranteed against disappearance by the mere power of per- 

 sistence possessed by the sexual determinants — a power which is 

 due, as we have seen, to the vast length of the period during 

 which amphimixis has existed. 



