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of animals and plants, or at least a great number of them, 
exhibit physiological characters—what are known as distinct 
species, structurally, being for the most part either altogether 
incompetent to breed one with another; or if they breed, 
the resulting mule, or hybrid, is unable to perpetuate its race 
with another hybrid of the same kind. 
A true physical cause is, however, admitted to be such 
only on one condition—that it shall account for all the 
phenomena which come within the range of its operation. 
If it is inconsistent with any one phenomenon, it must be 
rejected; if it fails to explain any one phenomenon, it is so 
far weak, so far to be suspected ; though it may have a per- 
fect right to claim provisional acceptance. 
Now, Mr. Darwin’s hypothesis is not, so far as I am aware, 
inconsistent with any known biological fact; on the con- 
trary, if admitted, the facts of Development, of Comparative 
Anatomy, of Geographical Distribution, and of Palzontology, 
become connected together, and exhibit a meaning such as 
they never possessed before; and I, for one, am fully con- 
vinced, that if not precisely true, that hypothesis is as near 
an approximation to the truth as, for example, the Coperni- 
can hypothesis was to the true theory of the planetary 
motions. 
But, for all this, our acceptance of the Darwinian hypo- 
thesis must be provisional so long as one link in the chain of 
evidence is wanting; and so long as allthe animals and plants 
certainly produced by selective breeding from a common stock 
are fertile, and their progeny are fertile with one another, 
that link will be wanting. For, so long, selective breeding will 
not be proved to be competent to do all that is required of 
it to produce natural species. 
I have put this conclusion as strongly as possible before 
the reader, because the last position in which I wish to find 
myself is that of an advocate for Mr. Darwin’s, or any other 
views—if by an advocate is meant one whose business it is 
