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 



