MR. HERBERT SPENCER. 331 



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 domesti- 

 cated animals, and in the several races of men, these 

 changes have uniformly 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 

 that it is a matter of dispute whether some of these 

 modified forms are varieties or modified species. 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 habitually gratified, and 

 the weakening of those habitually curbed ; the deve- 

 lopment 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 nature there is at work a modi- 

 fying 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 appearance, would produce in the millions 

 of years, and under the great varieties of condition 

 which geological records imply, any amount of change." 



