432 THE CAUSES OF THE xt 
parts of Bengal, into the dry, cold, bitter steppes 
of Siberia, into a latitude of 50°,—so that they may 
even prey upon the reindeer. These tigers have 
exceedingly different characteristics, but still they 
all keep their general features, so that there is no 
doubt as to their being tigers. The Siberian 
tiger has a thick fur, a small mane, and a longi- 
tudinal stripe down the back, while the tigers of 
Java and Sumatra differ in many important re- 
spects from the tigers of Northern Asia. So lions 
vary ; so birds vary; and so, if you go further back 
and lower down in creation, you find that fishes 
vary. In different streams, in the same country 
even, you will find the trout to be quite different 
to each other and easily recognisable by those who 
fish in the particular streams. There is the same 
differences in leeches ; leech collectors can easily 
point out to you the differences and the peculiari- 
ties which you yourself would probably pass by ; 
so with fresh-water mussels ; so, in fact, with every 
animal you can mention. 
In plants there is the same kind of variation. . 
Take such a case even as the common bramble. 
The botanists are all at war about it; some of 
them wanting to make out that there are many 
species of it, and others maintaining that they are 
but many varieties of one species; and they can- 
not settle to this day which is a species and which 
is a variety ! | 
So that there can be no doubt whatsoever that 
