106 Evolution and Adaptation 



a few exceptions, polymorphic in other countries, and like- 

 wise, judging from Brachiopod shells, at former periods of 

 time. These facts are very perplexing, for they seem to show 

 that this kind of variability is independent of the conditions 

 of life. I am inclined to suspect that we see, at least in some 

 of these polymorphic genera, variations which are of no 

 service or disservice to the species, and which consequently 

 have not been seized on by selection to act on and accumulate, 

 in the same manner as man accumulates in any given direc- 

 tion individual differences in his domesticated productions. 

 These individual differences generally affect what naturalists 

 consider unimportant parts ; but I could show by a long cata- 

 logue of facts, that parts which must be called important, 

 whether viewed under a physiological or classificatory point 

 of view, sometimes vary in the individuals of the same species. 

 I am convinced that the most experienced naturalist would 

 be surprised at the number of cases of variability, even in 

 important parts of structure, which he could collect on good 

 authority, as I have collected, during a course of years." 



After pointing out that naturalists have no definite stand- 

 ard to determine whether a group of individuals is a variety 

 or a species, Darwin makes the highly important admissions 

 contained in the following paragraph : " Hence, I look at indi- 

 vidual differences, though of small interest to the systematist, 

 as of the highest importance for us, as being the first steps 

 toward such slight varieties as are barely thought worth re- 

 cording in works on natural history. And I look at varieties 

 which are in any degree more distinct and permanent, as 

 steps toward more strongly marked and permanent varieties ; 

 and at the latter, as leading to subspecies, and then to species. 

 The passage from one stage of difference to another may, in 

 many cases, be the simple result of the nature of the organism 

 and of the different physical conditions to which it has long 

 been exposed ; but with respect to the more important and 

 adaptive characters, the passage from one stage of difference 



