OTHER THEORIES OF SPECIES-FORMING 273 



successful mating. Such conditions are readily observable between 

 closely allied species. Again, the prevention of intercrossing may 

 result from the appearance of a lowered interfertility between the 

 variant individuals and those of the parent-stock. If individuals 

 varying in the same direction were even slightly more fertile inter se 

 than those varying in different directions there would be a progressive 

 tendency in a series of generations for the varying individuals to 

 diverge more and more markedly, and ultimately to become practi- 

 cally sterile except with members of their own group. 



That environmental changes do frequently affect the fertility of 

 animals is seen when wild animals are kept in confinement. Rela- 

 tively few wild animals breed in captivity. Such a lowering of fer- 

 tility as the result of environmental changes might restrict crossing 

 between unlike forms, while permitting it among the like ones. 



Summary on isolation theories. — There is a great divergence of 

 opinion as to the importance of isolation as a causal factor in species- 

 forming. Some writers, such as D. S. Jordan and V. L. Kellogg, con- 

 sider isolation an indispensable, and therefore primary, factor; others, 

 especially geneticists, almost ignore it as an effective factor. Still 

 others, like the present writer, take a middle ground and conclude 

 that isolation, especially geographic isolation, has helped greatly in the 

 segregation and establishment of well-defined groups such as species 

 or varieties, the latter developing into the former after prolonged 

 isolation and the addition of new variations. Isolation theories, how- 

 ever, have no light to shed upon the difficult problem of adaptation, 

 and it is here that isolation is auxiliary to natural selection. 



THEORIES ALTERNATIVE TO NATURAL SELECTION 



The three theories that have been offered by their authors as sub- 

 stitutes for natural selection are: 



1. Theory of the inheritance of acquired characters commonly called 

 Lamarckism: This theory has been outlined in the chapter on the 

 history of evolution (pp. 19 ff.). It will again be dealt with in con- 

 siderable detail in chapter xxii. For the present, then, we may pass 

 by this theory without further comment. 



2. The orthogenesis theories: These theories have already been 

 presented in sufficient detail for our purposes in chapter ii (pp. t,^ ff.). 



3. The mutation theory of Hugo De Vries: This theory has been 

 dealt with in chapter ii, and will be discussed in further detail in 

 chapter xxiv. 



