186 Hybridism with Reference to the [April, 
from individuals of any other species. This combined like- 
ness and unlikeness was, however, and is still, very difficult 
to estimate, whence in many cases there arose great differ- 
ence of opinion as to the number of species of some parti- 
cular group inhabiting any given country. This very faCt, 
by the way, contributed no little to undermine, in the minds 
of careful inquirers, the notion of the absolute distin<5tness 
of species. To supplement the morphological boundary 
between species and species the so-called “ physiological 
test ” was appealed to. If two individuals of opposite sexes 
were seen in copula, this observation was taken for proof 
positive that they belonged to the same species. In like 
manner it was held that a brood of young animals must 
belong to the same species as their mother, or as both their 
parents if discoverable. 
It must be admitted, however, that for multitudes, espe- 
cially of the smaller animals, this so-called “ physiological 
test ” has not the slightest value. In how many, or rather 
how few, cases can we trace the entire series from the 
copulation of the parent animals to the appearance of the 
young ! 
Another point in the creed of the Old School was the 
broad, absolute distinction between “ species,” “ variety,” 
and “race.” Species was a something primordial and abso- 
lute; race, a something formed by change of climate, diet, 
or other surroundings. Races might pass into each other ; 
species, however structurally similar, never. Fruitful inter- 
course was possible between individuals of different races, 
but not between those of different species. 
I shall not, I think, be open to the charge of misrepre- 
senting the views of the Cuvierians on the question of 
hybridism if I put forward the following propositions, which 
are all, explicitly or implicitly, contained in their writings, 
and which are, indeed, the logical outcome of the doCtrine 
of the Immutability of Species ; — 
I. Without human interference sexual intercourse never 
takes place between individuals of different 
species. 
II. If by dint of such interference connection does hap- 
pen, it is normally unfruitful. 
III. In the exceptional cases where a hybrid is produced 
it is barren or impotent, and incapable of repro- 
duction. 
All these propositions can, I think, be successfully con- 
troverted. 
