98 READINGS IN EVOLUTION, -GENETICS, AND EUGENICS 
enable us to give a rational account not only of the peculiarities of 
form and structure presented by animals and plants, but also of their 
grouping together in certain areas, and their general distribution over 
the earth’s surface. 
“Tn the absence of any exact knowledge of the facts of distribution, 
a student of the theory of evolution might naturally anticipate that all 
groups of allied organisms would be found in the same region, and that, 
as he travelled farther and farther from any given centre, the forms 
of life would differ more and more from those which prevailed at the 
starting-point, till, in the remotest regions to which he could penetrate, 
he would find an entirely new assemblage of animals and plants, 
altogether unlike those with which he was familiar. He would also 
anticipate that diversities of.climate would always be associated with a 
corresponding diversity in the forms of life. 
“Now these anticipations are to a considerable extent justified. 
Remoteness on the earth’s surface is usually an indication of diversity 
in the fauna and flora, while strongly contrasted climates are always 
accompanied by a considerable contrast in the forms of life. But 
this correspondence is by no means exact or proportionate, and the 
converse propositions are often quite untrue. Countries which are 
near to each other often differ radically in their animal and vegetable 
productions; while similarity of climate, together with moderate 
geographical proximity, are often accompanied by marked diversi- 
ties in the prevailing forms of life. Again, while many groups of 
animals—genera, families, and sometimes even orders—are confined 
to limited regions, most of the families, many genera, and even 
some species are found in every part of the earth. An enumeration 
of a few of these anomalies will better illustrate the nature of the 
problem we have to solve. 
“As examples of extreme diversity, notwithstanding geographical 
proximity, we may adduce Madagascar and Africa, whose animal and 
vegetable productions are far less alike than are those of Great Britain 
and Japan at the remotest extremities of the great northern continent; 
while an equal, or perhaps even a still greater, diversity exists between 
Australia and New Zealand. On the other hand, Northern Africa 
and South Europe, though separated by the Mediterranean Sea, have 
faunas and floras which do not differ from each other more than do 
the various countries of Europe. Asa proof that similarity of climate 
and general adaptability have had but a small part in determining the 
forms of life in each country, we have the fact of the enormous increase 
