20 Darwin, and after Darwin. 
have been lost to observation. And this possibility 
becomes little less than a certainty when we note the 
next consideration which I have to adduce, namely, 
that in all their systematic divisions of plants and 
animals in groups higher than species— such as genera, 
families, orders, and the rest—naturalists have at all 
times recognised the fact that the one shades off into 
the other by such imperceptible gradations, that it 
is impossible to regard such divisions as other than 
conventional. It is important to remember that this 
fact was fully recognised before the days of Darwin. 
In those days the scientifically orthodox doctrine 
was, that although species were to be regarded as 
fixed units, bearing the stamp of a special creation, 
all the higher taxonomic divisions were to be con- 
sidered as what may be termed the artificial creation 
of naturalists themselves. In other words, it was 
believed, and in many cases known, that if we could 
go far enough back in the history of the earth, we 
should everywhere find a tendency to mutual ap- 
proximation between allied groups of species ; so that, 
for instance, birds and reptiles would be found to be 
drawing nearer and nearer together, until eventually 
they would seem to become fused in a single type; 
that the existing distinctions between herbivorous 
and carnivorous mammals would be found to do like- 
wise ; and so on with all the larger group-distinctions, 
at any rate within the limits of the same sub-kingdoms. 
But although naturalists recognised this even in the 
pre-Darwinian days, they stoutly believed that a 
great exception was to be made in the case of species. 
These, the lowest or initial members of their taxo- 
nomic series, they supposed to be permanent—the 
