THE BEGINNINGS OF LIFE. 579 



its bearings upon the shapes and modes of growth 

 of plants of different kinds, and is applicable even 

 to their most minute details of structure. Amongst 

 animals, however, the indirect influence of change in 

 external conditions (that which acts by inducing modi- 

 fied function, or c use and disuse') is also frequently 

 recognizable 1 this mode of operation being favoured 

 by the fact that the functions of animals are more 

 differentiated than those of plants. As Mr. Spencer says, 

 in order that c a new external action may be met by a 

 new internal action, it is needful that it shall either con- 

 tinuously or frequently be borne by the individuals of the 

 species, without killing or injuring them V If the new 

 influence act immediately upon the nutrition of the 

 organism in a way which we are unable to explain (as is 

 so frequently the case amongst plants), we regard it as 

 a case of the direct influence of change in conditions ; 

 whilst if the change acts (as is frequently, though by 

 no means invariably, the case amongst animals) in 

 more slowly bringing about some obvious difference 

 in function, then the difference of function is gene- 

 rally credited with bringing about the variation, and 

 the action of the change in conditions is at least said 

 to be c indirect.' Changes brought about by c use and 

 disuse, 5 in reality, therefore, belong to the same cate- 

 gory as those other changes which are said to be 



1 ' Principles of Biology,' vol. i. p. 439. 



2 Loc. cit., vol. i. p. 442. 



P p 2 



