992 DISTRIBUTION AND ZXTIOLOGY 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 + 2; 
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 +2 and A will be 
filled up by a series of forms in which # 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, primé facie, 
they are of distinct species ; yet if a large collection 
shows us that the interval between A and A + 1 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 be con- 
