3S0 INTBOD^JCTIO^' TO ZOOLOGY. 



above, whore the several orders follow each in regular succes- 

 sion, to convey an idea of the affinities which sometimes con- 

 nect families that belong to orders widely separated in the scale. 

 The same difficulty presents itself in every extensive assemblage 

 of animals, showing, as has ah-eady been remarked (p. 28), that 

 " the chain of beings of which the poet has sung, has no real 

 existence in natui-e." 



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 " Phy- 

 sical Atlas," as the basis of their calculations respecting the 

 proportionate number of the species. The species described 

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

 recorded as Irish, to little more than one-thii-d of that 

 number. t 



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, &c., which are scat- 

 tered throughout elementary works in general use. Our object 

 shall rather be to point out how the different orders are char- 

 acterized, and in what manner they are distributed. 



With the laws affijcting 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 hmits 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, Sir Charies Lyell 

 observes there are " several distinct zones of indigenous mam- 

 malia, 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 



• Professor Bell's British Quadrupeds. 



t Thompson's Report on the Fauna of Ireland. 



