122 PIONEERS OF EVOLUTION. 



has effected, and is effecting, decided changes in all 

 organisms subject to modifying influences. . . . 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, domesticated animals, and in the several races 

 of men, such alterations have taken place. They 

 can show that the degrees of difference so produced 

 are often, as in dogs, greater than those on which 

 distinctions of species are in other cases founded. 

 They can show, too, that the changes daily taking 

 place in ourselves the facility that attends long 

 practice, and the loss of aptitude that begins when 

 practice ceases the strengthening of passions habit- 

 ually gratified, and the weakening of those habitually 

 curbed the development of every faculty, bodily, 

 moral, or intellectual, according to the use made of 

 it are all explicable on this same principle. And 

 thus they can show that throughout all organic na- 

 ture there is at work a modifying influence of the 

 kind they assign as the cause of these specific dif- 

 ferences; an influence which, though slow in its 

 action, does, in time, if the circumstances demand it, 

 produce marked changes an influence which, to all 

 appearance, would produce in the millions of years, 

 and under the great varieties of condition which geo- 

 logical records imply, any amount of change." 



This quotation shows, as perhaps no other refer- 

 ence might show, how, by the middle of the present 

 century, science was trembling on the verge of dis- 



