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 hi the same direction were even slightly more fertile inter se 

 than those varying hi different directions there would be a progressive 

 tendency hi 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 hi confinement. Rela- 

 tively few wild animals breed hi 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 hi 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 hi 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 hi the chapter on the 

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

 siderable detail hi 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 hi sufficient detail for our purposes hi chapter ii (pp. 33 ff.). 



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

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

 chapter xxiv. 



