28 Darwin, and after Darwin. 
indicating natural affinities, the less is its value to the 
creatures presenting it. 
Enough has now been said to show three things. 
First that long before the theory of descent was enter- 
tained by naturalists, naturalists perceived the fact of 
natural affinities, and did their best to construct a 
natural system of classification for the purpose of ex- 
pressing such affinities. Second, that naturalists had 
a kind of instinctive belief in some one principle run- 
ning through the whole organic world, which thus 
served to bind together organisms in groups subor- 
dinate to groups—that is, into species, genera, orders, 
families, classes, sub-kingdoms, and kingdoms. Third, 
that they were not able to give any very intelligible 
reason for this faith that was in them ; sometimes 
supposing the principle in question to be that of a 
supernatural plan of organization, sometimes regarding 
itas dependent on conditions of physiology, and some- 
times not attempting to account for it at all. 
Of course it is obvious that the theory of descent 
furnishes the explanation which is required. For it is 
now evident to evolutionists, that although these older 
naturalists did not know what they were doing when 
they were tracing these lines of natural affinity, and 
thus helping to construct a natural classification—I say 
it is now evident to evolutionists that these naturalists 
were simply tracing the lines of genetic relationship. 
The great principle pervading organic nature, which 
was seen so mysteriously to bind the whole creation 
together as in a nexus of organic affinity, is now easily 
understood as nothing more or less than the prin- 
ciple of Heredity. Let us, therefore, look a little 
more closely at the character of this network, in 
