525 
The first argument which I propose to consider is that 
drawn from the geographical distribution of animals and 
plants, and from the relation which sometimes exists 
between the past and the present inhabitants of any given 
locality. From these phenomena it is inferred that exist 
ing species “ came into existence where they now occur 
by way of genetic descent, with perpetual migration and 
correlative modification,” and that “ every species has 
come into existence coincident both in space and time 
with a pre-existing and closely-allied species.” In short, 
the argument is an argument to show that the transmu 
tation of species has taken place in the past even as it 
is now taking place in the present. 
The change of conditions which requires a readjustment 
of the organism to adapt itself may arise either through 
the transference of a species to other spheres, or through 
the change of conditions which may occur in the same 
locality from time to time. In connection with the former 
problem, we have especially to consider the circumstances 
under which the distribution has taken place, whether of 
conscious emigrants or of the passive victims of certain 
means of dispersal. We have now to consider how far this 
geographical distribution has been favourable to the trans 
mutation of species by means of Natural Selection. 
Assuming that a species started from a given locality, 
it would naturally spread in those directions where there 
was an opening, and move on those lines in which there was 
an unbroken and continuous connection. The principal 
method of distribution would therefore be by a process of 
emigration whenever a favourable outlet could be found. 
The argument for Organic Evolution, drawn from geo 
graphical distribution, is based upon the fact that along 
such lines of unbroken communication we find a series of 
allied species adapted to their different conditions. There