EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 99 
of rabbits and pigs in Australia and New Zealand, of horses and cattle 
in South America, and of the common sparrow in North America, 
though in none of these cases are the animals natives of the 
countries in which they thrive so well. And lastly, in illustration of - 
the fact that allied forms are not always found in adjacent regions, 
we have the tapirs, which are found only on opposite sides of the 
globe, in tropical America and the Malayan Islands; the camels of 
the Asiatic deserts, whose nearest allies are the Ilamas 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 abundantly 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, per- 
fectly 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 productions of 
remote countries resemble each other, there is almost always conti- 
nuity 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 
permanence. When a group of animals inhabits two countries 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, comparatively 
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 
