VARIETIES AND TRANSITIONAL FORMS. 293 
cluded that A and A + m 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 ~ is a small and unimportant differ- 
ence, such as of average size, colour, or ornamenta- 
tion, it may be fairly held that A and A + m 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 thatthe 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: 
