292 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



small, and so inconstant, that they lie within the 

 probable limits of individual variation. 



Observation further acquaints us with the fact, that, 

 sometimes, an individual member of a species may 

 exhibit a more or less marked variation, which is pro- 

 pagated through all the offspring of that individual, 

 and may even become intensified in them. And, in 

 this manner, a variety, or race, is generated within the 

 species; which variety, or race, if nothing were known 

 respecting its origin, might have every claim to be 

 regarded as a separate morphological species. The 

 distinctive characters, of a race, however, are rarely 

 equally well marked in all the members of the race. 

 Thus suppose the species A to develope the race A + x ; 

 then the difference x is apt to be much less in some 

 individuals than in others ; so that, in a large suite of 

 specimens, the interval between A + x and A will be 

 filled up by a series of forms in which x gradually 

 diminishes. 



Finally, it is a matter of observation that modification 

 of the physical conditions under which a species lives 

 favours the development of varieties and races. 



Hence, in the case of two specimens having respec- 

 tively the characters A and A + n, although, prima facie, 

 they are of distinct species ; yet if a large collection 

 shows us that the interval between A and A + n is filled 

 up by forms of A having traces of n, and forms of A + n 

 in which n becomes less and less, then it will he con- 



