RISE OF EVOLUTIONARY THOUGHT 4 T 9 



developmental hypothesis, saying that even if its supporters 

 could "merely show that the production of species by the 

 process of modification is conceivable, they would be in a 

 better position than their opponents. But they can do much 

 more than this; the}' 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 imply, 

 any amount of change." 



"It is impossible," says Marshall, u 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 



