I.] FACTORS IN DISTRIBUTION. 5 



for the existence of the former, while the total quantity of heat 

 suffices for the latter. In other words, there is a low summer 

 temperature combined with a high total sum of heat. 



Among the secondary causes affecting distribution, humidity, 

 according to the same observer, may occupy the first 

 place. "Humidity," he writes, " governs details of 

 distribution of numerous species of plants, reptiles and birds, and 

 of a few species of mammals, within the several temperature-zones. 

 . . . Humidity and other secondary causes determine the presence 

 or absence of particular species in particular localities within their 

 appropriate zones, but temperature predetermines the possibili- 

 ties of distribution ; it fixes the limits beyond which species 

 cannot pass ; it defines broad trans-continental belts within 

 which certain forms may thrive if other conditions permit, but 

 outside of which they cannot exist, be the other conditions never 

 so favourable." 



Important as the influence of temperature and, in a smaller 

 degree, that of humidity, has undoubtedly been in other Fac 

 determining the distributional limits of the species tors in Distri- 



f. , , , . , , . . bution. 



or genera ot animals and plants now inhabiting 

 particular countries, and large as has been the part played by the 

 glacial epoch in producing the present condition of things, it is 

 evident that temperature has been by no means the only, even if 

 it be the chief factor in distribution. In the first place there are 

 several species, more especially among the carnivorous mammals, 

 which seem quite independent of both station and temperature, 

 the New World puma ranging from Patagonia to Canada, while 

 the tiger inhabits alike the burning jungles of India and Burma, 

 and the Arctic tundras of Siberia. Striking as such cases are, they 

 are, however, to be regarded merely as examples of the individual 

 adaptability of certain species, which, like the carnivores named, 

 are able to obtain suitable food in any part of the world, and 

 they do not throw any discredit on the power of temperature as 

 a controlling factor in animal distribution generally. 



Of the utmost importance in this respect are the changes 

 which the surface of the globe itself has undergone in past 

 epochs, whereby continents that are now more or less completely 

 sundered from one another were formerly connected, while what 



