PRESIDENTS ADDRESS. 409 
whole armies of workers for centuries to come, even if it be 
possible to ever bridge over the gaps which exist owing to the 
extermination of intermediate types which have left no fossil 
remains. In the meantime, in the absence of data by means of 
which reasonably direct descent can be established, we must be 
careful, lest in our enthusiastic endeavours to present a completed 
pedigree, we too closely resemble the efforts of the College of Arms, 
which, I believe, are always satisfactorily perfect. 
The doctrine of evolution, as well as supplying anatomists with 
a system, afforded by its principle of natural selection a new inter- 
pretation of the teleological arguments, the validity of which were 
necessarily so convincing to every morphologist. Jor, whereas, in 
the old teleology, adaptation of structure to function was a sufli- 
cient explanation of the existence of structural variation, Darwin 
showed that it is possible to go further than this, and explain 
how the adaptation exists and continues to exist. He showed that 
adaptation itself might be the effect of a sum total of causes, or, 
in other words, to the operation of natural selection. 
By showing morphologists that, although they might justly 
provisionally arrange their facts in a teleological category, adapta- 
tion itself is capable of a casual interpretation, Darwin, once for 
all, disposed of the inconsistency which had arisen between the 
points of view of students of form and function. Natural selec- 
tion is explicable without going outside the material laws of the 
inorganic world, consequently adaptation of structure to function 
may, after all, ‘be regulated blindly, according to the laws of 
necessity” —a view of vital phenomena, which, when put forward 
by Schwann, in 1839, appeared so impossible to anatomists. 
Having accumulated an overwhelming mass of evidence to estab- 
lish the doctrine of evolution, morphologists have, during the last 
ten years, been discussing whether the factors, natural selection or 
sexual selection, put forward by Darwin, are adequate to explain 
the origin of species, or whether it is necessary to find some further 
factor. 
Whilst the majority of biologists fully agree that natural 
selection is sufficient to produce an evolutionary advance in type, 
they feel that this factor alone would only account for a monotypic 
evolution as long as free intercrossing can take place. The result 
of free crossing would level down all variations. Cross-breeding 
and its consequences may be prevented by geographical and 
climatic isolation just as it is intentionally prevented by artificial 
selection in breeding, but geographical and climatic isolation 
cannot, however, explain the existence of distinct species 
inhabiting the same area. 
The fact that most animals and plants of different species are 
relatively or absolutely infertile one with another, has frequently 
been advanced by opponents of natural selection in favour of 
