DIVERGENCE 263 



of the same kind vary under natural conditions. Con- 

 cerning this, he says : 



"The many slight differences which appear in the offspring 

 from the same parents, or which it may be presumed have thus 

 arisen, from being observed in the same locality, may be called 

 individual differences. No one supposes that all the individuals 

 of the same species are cast in the same actual mould. These 

 individual differences are of the highest importance to us, for they 

 are often inherited, as must be familiar to everyone; and they 

 thus afford materials for natural selection to act on and accumulate 

 in the same manner as man accumulates in any given direction 

 individual differences in his domesticated productions." 



"I look at individual 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 recording 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 sub-species, 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 existence to another may be safely 

 attributed to the cumulative action of natural selection, hereafter 

 to be explained, and to the effects of the increased use or disuse 

 of parts. A well-marked variety may therefore be called an 

 incipient species; but whether this belief is justifiable must be 

 judged by the weight of the various facts and considerations to be 

 given throughout this work" [Origin of Species]. 



"Varieties cannot be distinguished from species except, first, 

 by the discovery of intermediate linking forms; and, secondly, by a 

 certain indefinite amount of difference between them for two forms, 

 if differing very little, are generally ranked as varieties, notwith- 

 standing that they cannot be closely connected; but the amount of 

 difference considered necessary to give to any two forms the rank 

 of species cannot be defined." * 



"I must make a few preliminary remarks to show how the 

 struggle for existence bears on natural selection. It has been 

 seen . . . that among organic beings in a state of nature there is 

 some individual variability; indeed I am not aware that it has 

 ever been disputed. It is immaterial for us whether a multitude 

 of doubtful forms be called species or sub-species or varieties . . . 

 if the existence of any well-marked varieties be admitted. But 

 the mere existence of individual variability, and of some few 

 well-marked varieties, though necessary as the foundation for this 

 work, helps us but little in understanding how species arise in 

 nature." 



"If under changing conditions of life organic beings present 

 individual differences in almost every part of their structure, and 

 this cannot be disputed; if there be, owing to their geometrical 



