Prof. Huxley on the Animals between Birds and Reptiles. 67 
immeasurable time, by the operation of causes more or less similar 
to those which are at work at the present day. 
Perhaps this doctrine of evolution is not maintained consciously 
and in its logical integrity by a very great number of persons *. But 
many hold particular applications of it without committing them- 
selves to the whole; and many, on the other hand, favour the ge- 
neral doctrine without giving an absolute assent to its particular 
applications, 
Thus, one who adopts the nebular hypothesis in astronomy, or is 
a uniformitarian in geology, or a Darwinian in biology, is so far an 
adherent of the doctrine of evolution. 
And, as I can testify from personal experience, it is possible to 
have a complete faith in the general doctrine of evolution and yet to 
hesitate in accepting the nebular, or the uniformitarian, or the Dar- 
winian hypotheses in all their integrity and fulness; for many of 
the objections which are brought against these various hypotheses 
affect them only, and, even if they be valid, leave the general doc- 
trine of evolution untouched. 
On the other hand, it must be admitted that some arguments which 
are adduced against particular forms of the doctrine of evolution 
would very seriously affect the whole doctrine if they were proof 
against refutation. 
For example, there is an objection which I see constantly and 
confidently urged against Mr. Darwin’s views, but which really strikes 
at the heart of the whole doctrine of evolution, so far as it is applied 
to the organic world. 
It is admitted on all sides that existing animals and plants are 
marked out by natural intervals into sundry very distinct groups: 
insects are widely different from fish, fish from reptiles, reptiles 
from mammals, and so on. And out of this fact arises the very 
pertinent objection, How is it, if all animals have proceeded by 
gradual modification from a common stock, that these great gaps 
exist ? 
We, who believe in evolution, reply that these gaps were once 
non-existent ; that the connecting forms existed in previous epochs 
of the world’s history, but that they have died out. 
Naturally enough, then, we are asked to produce these extinct 
forms of life. Among the innumerable fossils of all ages which 
exist, we are asked to point to those which constitute such connect- 
ing forms. 
Our reply to this request is, in most cases, an admission that such 
forms are not forthcoming; and we account for this failure of the 
needful evidence by the known imperfection of the geological record. 
We say that the series of formations with which we are acquainted 
is but a small fraction of those which have existed, and that between 
those which we know there are great breaks and gaps. 
* The only complete and systematic statement of the doctrine with 
which I am acquainted is that contained in Mr. Herbert Spencer's ‘Sys- 
tem of Philosophy,’ a work which should be carefully studied by all who 
desire to know whither scientific thought is tending. ze 
