122 ZOOLOGICAL GEOGRAPHY. [part hi. 



any other. Though not very rich, and having many disadvan- 

 tages of climate and of physical condition, it is yet sufficiently 

 well characterized in its zoological features to rank as one of 

 the well-marked primary divisions of the earth's surface. 



There is one other consideration bearing on this question 

 which should not be lost sight of In establisliing our regions 

 we have depended wholly upon tlieir noio possessing a sufficient 

 number and variety of animal forms, and a fair proportion of 

 peculiar types ; but when the validity of our conclusion on these 

 grounds is disputed, we may supplement the evidence by an 

 appeal to the past history of the region in question. In this 

 case we find a remarkable support to our views. During the 

 whole Tertiary period, North America was, zoologically, far 

 more strongly contrasted with South America than it is now ; 

 while, during the same long series of ages, it was always clearly 

 separated from the Eastern hemisphere or the Pahearctic region 

 by the exclusive possession of important families and numerous 

 genera of Mammalia, as shown by our summary of its extinct 

 fauna in Chapter VII. Not only may we claim North America 

 as now forming one of the great zoological regions, but as having 

 continued to be one ever since the Eocene period. 



Insects. 



In describing the Palsearctic and Neotropical regions, many of 

 the peculiarities of the insect-fauna of this region have been 

 incidentally referred to ; and as a tolerably full account of the 

 distribution of the several families is given in the Fourth Part 

 of our work (Chapter XXI.), we shall treat the subject very 

 briefly here. 



Le.'pido'pUra. — The butterflies of the Nearctic region have 

 lately been studied with much assiduity, and we are now able 

 to form some idea of their nature and extent. Nearly 500 

 species belonging to about 100 genera have been described ; 

 showing that the region, which a few years ago was thought to 

 be very poor in species of butterflies, is really much richer than 

 Europe, and probably about as rich as the more extensive Palse- 

 arctic region. There is, however, very little speciality in tlie 



