226 Darwin, and after Darwin. 



As we shall have to consider the important principle 

 of isolation more fully on a subsequent occasion, 

 I need not deal with it in the present connexion, 

 further than to remark that in this principle we have 

 what appears to me a full and adequate condition to 

 the rise and continuance of specific characters which 

 need not necessarily be adaptive characters. And, when 

 we come to consider the facts of isolation more closely, 

 we shall find superabundant evidence of this having 

 actually been the case. 



V. Laws of Growth. 



Under this general term Darwin included the opera- 

 tion of all unknown causes internal to organisms 

 leading to modifications of form or structure — such 

 modifications, therefore, appearing to arise, as he 

 says " spontaneously," or without reference to utility. 

 That he attributed no small importance to the opera- 

 opportunity for perpetuation will have been given to any congenital 

 variations which may happen to arise. Should any of these be pronounced 

 variations, it would afterwards be ranked as a specific character. 

 I do not myself thinlc that this is the way in which indifferent specific 

 characters usually originate. On the contrary, I believe that their 

 origin is most frequently due to the influence of isolation on the average 

 characters of the whole population, as briefly stated in the text. But 

 here it seems worth while to notice this possibility of their occa- 

 sionally arising as merely individual variations, afterwards perpetuated 

 by any of the numerous isolating conditions which occur in nature. 

 For, if this can be the case with a varietal form so extreme as to border 

 on the monstrous, much more can it be so with such minute differences 

 as frequently go to constitute specific distinctions. It is the business of 

 species-makers to search out such distinctions, no matter how trivial, 

 and to record them as " specific characters." Consequently, wherever 

 in nature a congenital variation happens to arise, and to be perpetuated 

 by the force of heredity alone under any of the numerous forms of isola- 

 tion which occur in nature, there will be a case analogous to that of the 

 niata cattle. 



