396 TRANSACTIONS OF SECTION D. 
Jed to the formation of those more or less well-marked groups of organisms which 
we call species?’ We have to note in the first place that there is no unanimity 
of opinion amongst biologists as to what a species is. Lamarck insisted that 
nature recognises no such things as species, and a great many people at the 
present day are, I think, still of the same opinion. In practice, however, every 
naturalist knows that there are natural groups to which the vast majority of 
individuals can be assigned without any serious difficulty. Charles Darwin main- 
tained that such groups arose, under the influence of natural selection, through 
gradual divergent evolution and the extinction of intermediate forms. To-day 
we are told by de Vries that species originate as mutations which propagate 
themselves without alteration for a longer or shorter period, and by Lotsy that 
species originate by crossing of more or less distinct forms, though this latter 
theory leaves quite unsolved the problem of where the original forms that crossed 
with one another came from. 
I think a little reflection will convince us that the origin of species is a 
different problem from that of the cause of progressive evolution. We can 
hardly doubt, however, that Darwin was right in attributing prime importance 
to divergent evolution and the disappearance of connecting links. It is obvious 
that this process must give rise to more or less sharply separated groups of 
individuals to which the term species may be applied, and that the differences 
between these species must be attributed ultimately to differences in the response 
of the organism to differing conditions of the environment. It may be urged 
that inasmuch as different species are often found living side by side under 
identical conditions the differences between them cannot have arisen in this way, 
but we may be quite certain that if we knew enough of their past history we 
should find that their ancestors had not always lived under identical conditions. 
The case of flightless birds on oceanic islands is particularly instructive in 
this connection. The only satisfactory way of explaining the existence of such 
birds is by supposing that their ancestors had well developed wings, by the aid 
of which they made their way to the islands from some continental area. The 
conditions of the new environment led to the gradual disuse and consequent 
degeneration of the wings until they either became useless for flight or, in the 
case of the moas, completely disappeared. It would be absurd to maintain 
that any of the existing flightless birds are specifically identical with the 
ancestral flying forms from which they are descended, and it would, it appears to 
me, be equally absurd to suppose that the flightless species arose by mutation or 
by crossing, the same result being produced over and over again on different 
islands and in different groups of birds. This is clearly a case where the 
environment has determined the direction of evolution. 
In such cases there is not the slightest ground for believing that crossing has 
had anything whatever to do with the origin of the different groups to which the 
term species is applied; indeed the study of island faunas in general indicates 
very clearly that the prevention of crossing, by isolation, has been one of the 
chief factors in the divergence of lines of descent and the consequent multipli- 
cation of species, and Romanes clearly showed that even within the same 
geographical area an identical result may be produced by mutual sterility, which 
is the cause, rather than the result, of specific distinction. 
Species, then, may clearly arise by divergent evolution under changing 
conditions of the environment, and may become separated from one another by 
the extinction of intermediate forms. The environmental stimuli (including, of 
course, the body as part of its own environment) may, however, act in two dif- 
ferent ways : (1) Upon the body itself, at any stage of its development, tending 
to cause adaptation by individual selection of the most appropriate response; and 
(2) upon the germ-plasm, causing mutations or sudden changes, sports, in fact, 
which appear to have no direct relation whatever to the well-being of the 
organism in which they appear, but to be purely accidental. Such mutations 
are, of course, inherited, and, inasmuch as the great majority of specific 
characters appear to have no adaptive significance, it seems likely that mutation 
has had a great deal to do with the origin of species, though it may have had 
very little to do with progressive evolution. 
Similarly with regard to hybridisation, we know that vast numbers of distinct 
forms, that breed true, may be produced in this way, but they are simply 
due to recombinations of mutational characters in the process of amphimixis, and 
