ted THE ORIGIN OF SPECIES 49 
_attempts to express all kinds of resemblance between all 
; 
_ First crosses between forms known to be varieties, or 
_ sufficiently alike to be considered as varieties, and their mon- 
\ grel offspring, are very generally, but not quite universally, 
\fertile. Nor is this nearly general and perfect fertility sur- 
| prising, when we remember how liable we are to argue in a 
_ cirele with respect to varieties in a state of Nature ; and when 
we remember that the greater number of varieties have been 
produced under domestication by the selection of mere external 
| differences, and not of differences in the reproductive system. 
In all other respects, excluding fertility, there is a close general 
resemblance between hybrids and mongrels.”—Pp. 276—8. 
We fully agree with the general tenor of this 
weighty passage ; but forcible as are these argu- 
ments, and little as the value of fertility or 
infertility as a test of species may be, it must not 
be forgotten that the really important fact, so far 
as the inquiry into the origin of species goes, is, 
that there are such things in Nature as groups of 
animals and of plants, the members of which are in- 
capable of fertile union with those of other groups ; 
and that there are such things as hybrids, which 
are absolutely sterile when crossed with other 
hybrids. For, if such phenomena as these were 
exhibited by only two of those assemblages of 
living objects, to which the name of species 
(whether it be used in its physiological or in its 
morphological sense) is given, it would have to be 
accounted for by any theory of the origin of 
Species, and every theory which could not account 
for it would be, so far, imperfect. 
VOL. II E 
