44, THE ORIGIN OF SPECIES Ir 
to be obvious ; for if, as the result of spontaneou 
variation and of selective breeding, the progeny of — 
a common stock may become separated into groups - 
distinguished from one another by constant, not 
sexual, morphological characters, it is clear that 
the physiological definition of species is likely to 
clash with the morphological definition. No one 
would hesitate to describe the pouter and the 
tumbler as distinct species, if they were found fossil, | 
or if their skins and skeletons were imported, as 
those of exotic wild birds commonly are—and with- 
out doubt, if considered alone, they are good and 
distinct morphological species. On the other hand, — 
they are not physiological species, for they are 
descended from a common stock, the rock-pigeon. 
Under these circumstances, as it is admitted on 
all sides that races occur in Nature, how are we to 
know whether any apparently distinct animals are — 
really of different physiological species, or not, j 
seeing that the amount of "morphological difference 
‘is no_safe guide,’ 2 Is there any test of a physio- 
logical species? The usual answer of physiologists 
is in the affirmative. It is said that such a test is | 
to be found in the phenomena of hybridisation— ~ 
in the results of crossing races, as_ compared with 
the results of crossing species. 
So far as the evidence goes at present, in- 
dividuals, of what are certainly known to be mere — 
races produced by selection, however distinct they 
may appear to be, not only breed freely together, 
a _ 
