Summary 189 



of the distinctively new. Some important steps of 

 progress have been made, (a) It is clear that most of 

 the variations that count have a germinal origin; 

 they find expression in the body of the offspring; 

 but they begin as permutations and combinations or 

 changes in the hereditary "factors" or genes in the 

 germ-cells of the parent, (b) Mutations or discon- 

 tinuous variations are not uncommon, and when they 

 come they come to stay, (c) Variations are often 

 correlated with other variations, (d) In some cases 

 there is evidence of orthogenetic variation — that is 

 to say, steady change in some definite direction, (e) 

 In any case, a variation — if viable at all — must be 

 congruent with the organization already established. 

 15. The theory of Natural Selection suggests that 

 Nature's sifting of heritable variations in the strug- 

 gle for existence consistently results in the survival 

 of those variants that exhibit some character or 

 quality giving them an advantage over their neigh- 

 bors. The advantage may be a firmer foothold, better 

 food, greater success in evading enemies, longer life, 

 a larger family, more success in launching the off- 

 spring on the voyage of life, and so forth. It is inter- 

 esting to consider the evolution of sieves from age to 

 age and from type to type. The oldest sieve is the 

 quest for food; another not less primordial is the 

 physical environment with its vicissitudes ; then there 

 is the animate environment of competitors, neighbors, 

 partners, parasites; in some cases there is the gre- 

 garious or social life ; there is also the sieve of court- 

 ship, or, more generally, of preferential mating ; and 



