140 LAND MAMMALS IN THE WESTERN HEMISPHERE 



of such great continental islands as Great Britain, Java, 

 Sumatra, etc., on the one hand, and Australia, on the other. 

 The continental islands, which have but lately been detached 

 from the neighbouring main lands, are hardly more peculiar 

 zoologically than equal areas of the adjoining continents, 

 while the long-continued isolation of Australia has made it 

 the most peculiar region of the earth. Climatic changes, which, 

 as we saw in Chapter I, have indubitably taken place many 

 times, have also had a great effect in shifting the distribution 

 of mammals, which in its present form is the outcome of a 

 very long series of geographical and climatic changes, on the 

 one hand, and of adaptive changes in the animals themselves, 

 on the other. 



Of almost equal importance as a barrier is climate and 

 especially temperature. Not that similar climates can pro- 

 duce similar forms in separate areas. Regions of almost 

 exactly similar climate in Australia, Africa and South America 

 have totally different faunas, but, within continuous land-areas, 

 the most effective of barriers is temperature. This acts dif- 

 ferently in the case of limiting the northward spread of south- 

 ern forms and the southward spread of northern species. Dr. 

 Merriam's long study of this problem has led him to the con- 

 clusion that southern species are bounded on the north by the 

 temperature of the breeding season, in which the total quantity 

 of heat must reach a certain minimum, while "animals and 

 plants are restricted in southward distribution by the mean tem- 

 perature of a brief period covering the hottest part of the year." 

 On the Pacific coast there is a remarkable mingling in the same 

 areas of species which, east of the high mountains, are dis- 

 tributed in sharply separated zones. This is explained by the 

 mild and equable climate of the coastal belt, where the hottest 

 season of the year does not reach the limiting maximum for 

 the northern species, while the total quantity of heat in the 

 breeding season is sufficient to enable southern species to 

 thrive and maintain themselves. 



