396 INTRODUCTION TO ZOOLOGY. 



the above, where the several orders follow each in regular 

 succession, to convey an idea of the affinities which some- 

 times connect families that belong to orders widely separated 

 in the scale. The same difficulty presents itself in every 

 extensive assemblage of animals, showing, as has already 

 been remarked (p. 28) that " the chain of beings of which 

 the poet has sung, has no real existence in nature." 



The number of animals belonging to the class mammalia, 

 has been variously estimated, from 1149 to 1500; the latter 

 number is that adopted by the learned authors of the Physical 

 Atlas, as the basis of their calculations respecting the 

 proportionate number of the species. The species described 

 as British,* amount to between eighty and ninety, and those 

 recorded as Irish, to little more than one third of that 

 number.! 



In the limited space to which we are restricted, we shall 

 not attempt to introduce those anecdotes illustrative of the 

 habits of the Elephant, the Tiger, the Reindeer, <fec. which 

 are scattered throughout elementary works in general use. 

 Our object shall rather be to point out how the different orders 

 are characterised, and in what manner they are distributed. 



With the laws affecting their geographical distribution, we 

 are as yet but imperfectly acquainted. One of the most 

 obvious causes which limit the growth of vegetables, and the 

 range of animals within certain bounds, is temperature. Heat 

 and moisture stimulate the growth of plants, and wherever 

 vegetation is most luxuriant, there the land animals are most 

 abundant. They are confined within certain limits by the 

 intervention of seas and of continuous ranges of mountains. 

 But even when such obstacles do not exist, animals appear 

 subject to certain climatic conditions, and pass not the limits 

 which the Author of the Universe has fixed as the bounds of 

 their habitation. Thus in North America, Mr. Lyell observes 

 there' are "several distinct zones of indigenous mammalia, 

 extending east and west on the continent, where there are no 

 great natural boundaries running in the same direction, such 

 as mountain ridges, deserts, or wide arms of the sea, to check 

 the migration of species. The climate alone has been sufficient 

 to limit their range. The mammiferous fauna of New York, 

 comprising about forty species, is distinct from that of the 



* Professor Bell's British Quadruped?. 



t Thompson's Report on the Fauna of Ireland. 





