RISE OF EVOLUTIONARY THOUGHT 419 
developmental hypothesis, saying that even if its supporters 
could “merely show that the production of species by the 
process of modification 1s conceivable, they would be in a 
better position than their opponents. But they can do much 
more than this; they can show that the process of modifica- 
tion has affected and is affecting great changes in all organ- 
isms subject to modifying influences. ... They can show 
that any existing species, animal or vegetable, when placed 
under conditions different from its previous ones, imme- 
diately begins to undergo certain changes of structure fitting 
it for the new conditions. They can show that in successive 
generations these changes continue, until ultimately the 
new conditions become the natural ones. They can show 
that in cultivated plants and domesticated animals, and in the 
several races of men, these changes have uniformly taken 
place. They can show that the degrees of difference so pro- 
duced are often, as in dogs, greater than those on which dis- 
tinctions of species are in other cases founded. They can show 
that it is a matter of dispute whether some of these modified 
forms are varieties or modified species. And thus they can 
show that throughout all organic nature there is at work a 
modifying influence of the kind they assign as the cause of 
these specific differences; an influence which, though slow 
in its action, does in time, if the circumstances demand it, 
produce marked changes; an influence which, to all appear- 
ance, would produce in the millions of years, and under the 
great varieties of conditions which geological records implv, 
any amount of change.” 
“Tt is impossible,” says Marshall, “to depict better than 
this the condition prior to Darwin. In this essay there is full 
recognition of the fact of transition, and of its being due to 
natural influences or causes, acting now and at all times. 
Yet it remained comparatively unnoticed, because Spencer, 
like his contemporaries and predecessors, while advocating 
