328 Geographical Distribution of Animals 
view of the permanence of the oceans he accounts for the colonisation 
of outlying islands by further elaborating the views of Lyell and 
Darwin, especially in his fascinating Island Life, with remarkable 
chapters on the Ice Age, Climate and Time and other fundamental 
factors. His method of arriving at the degree of relationship of the 
faunas of the various regions is eminently statistical. Long lists of 
genera determine by their numbers the affinity and hence the source 
of colonisation. In order to make sure of his material he performed 
the laborious task of evolving a new classification of the host of 
Passerine birds. This statistical method has been followed by many 
authors, who, relying more upon quantity than quality, have obscured 
the fact that the key to the present distribution lies in the past 
changes of the earth’s surface. However, with Wallace begins the 
modern study of the geographical distribution of animals and the 
sudden interest taken in this subject by an ever widening circle of 
enthusiasts far beyond the professional brotherhood. 
A considerable literature has since grown up, almost bewildering 
in its range, diversity of aims and style of procedure. It is a chaos, 
with many paths leading into the maze, but as yet very few take us 
to a position commanding a view of the whole intricate terrain with 
its impenetrable tangle and pitfalls. 
One line of research, not initiated but greatly influenced by 
Wallace’s works, became so prominent as to almost constitute a 
period which may be characterised as that of the search by specialists 
for either the justification or the amending of his regions. As class 
after class of animals was brought up to reveal the secret of the true 
regions, some authors saw in their different results nothing but the 
faultiness of previously established regions; others looked upon 
eventual agreements as their final corroboration, especially when for 
instance such diverse groups as mammals and scorpions could, with 
some ingenuity, be made to harmonise. But the obvious result of 
all these efforts was the growing knowledge that almost every class 
seemed to follow principles of its own. The regions tallied neither in 
extent nor in numbers, although most of them gravitated more and 
more towards three centres, namely Australia, South America and 
the rest of the world. Still zoologists persisted in the search, and the 
various modes and capabilities of dispersal of the respective groups 
were thought sufficient explanation of the divergent results in trying 
to bring the mapping of the world under one scheme. 
Contemporary literature is full of devices for the mechanical 
dispersal of animals. Marine currents, warm and cold, were favoured 
all the more since they showed the probable original homes of the 
creatures in question. If these could not stand sea-water, they 
floated upon logs or icebergs, or they were blown across by storms ; 
