340 
DARWINISM 
CHAP. 
the Asiatic deserts, whose nearest allies are the llamas 
and alpacas of the Andes; and the marsupials, only found 
in Australia and on the opposite side of the globe, in 
America. Yet, again, although mammalia may be said to 
be universally distributed over the globe, being found abund¬ 
antly on all the continents and on a great many of the larger 
islands, yet they are entirely wanting in New Zealand, and in 
a considerable number of other islands which are, nevertheless, 
perfectly able to support them when introduced. 
Now most of these difficulties can be solved by means of 
well-known geographical and geological facts. When the pro¬ 
ductions of remote countries resemble each other, there is 
almost always continuity of land with similarity of climate 
between them. When adjacent countries differ greatly in 
their productions, we find them separated by a sea or strait 
whose great depth is an indication of its antiquity or per¬ 
manence. When a group of animals inhabits two coun¬ 
tries or regions separated by wide oceans, it is found that 
in past geological times the same group was much more 
widely distributed, and may have reached the countries it 
inhabits from an intermediate region in which it is now extinct. 
We know, also, that countries now united by land were 
divided by arms of the sea at a not very remote epoch; while 
there is good reason to believe that others now entirely 
isolated by a broad expanse of sea were formerly united and 
formed a single land area. There is also another important 
factor to be taken account of in considering how animals and 
plants have acquired their present peculiarities of distribution, 
— changes of climate. We know that quite recently a glacial 
epoch extended over much of what are now the temperate 
regions of the northern hemisphere, and that consequently 
the organisms which inhabit those parts must be, com¬ 
paratively speaking, recent immigrants from more southern 
lands. But it is a yet more important fact that, down to 
middle Tertiary times at all events, an equable temperate 
climate, with a luxuriant vegetation, extended to far within 
the arctic circle, over what are now barren wastes, covered 
for ten months of the year with snow and ice. The arctic 
zone has, therefore, been in past times* capable of supporting 
almost all the forms of life of our temperate regions; and we 
