VARIETIES AND TRANSITIONAL FORMS. 293 



eluded that A and A + n are races of one species and 

 not separate species. And this conclusion will be fortified 

 if A and A + n occupy different stations in the same 

 geographical area. 



Even when no transitional forms between A and A + n 

 are discoverable, if n is a small and unhnportant differ- 

 ence, such as of average size, colour, or ornamenta- 

 tion, it may be fairly held that A and A -\- n are mere 

 varieties ; inasmuch as experience proves that such 

 variations may take place comparatively suddenly; or 

 the intermediate forms may have died out and thus the 

 evidence of variation may have been effaced. 



From what has been said it follows that the groups 

 termed morphological species are provisional arrange- 

 ments, expressive simply of the present state of our 

 knowledge. 



We call two groups species, if we know of no tran- 

 sitional forms between them, and if there is no reason to 

 believe that the differences which they present are such 

 as may arise in the ordinary course of variation. But 

 it is impossible to say whether the progress of in- 

 quiry into the characters of any group of individuals 

 may prove that what have hitherto been taken for mere 

 varieties are distinct morphological species ; or whether, 

 on the contrary, it may prove that what have hitherto 

 been regarded as distinct morphological species are mere 

 varieties. 



What has happened in the case of the crayfish is this : 



