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. 33 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. : 
