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 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 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 hov/ animals a ad 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 tinies 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 



