Works on Biology, etc. 



other great divisions; while at the same time it helped to 

 explain the apparent contradictions in the distribution of 

 land animals. Some years lat^r he wrote : 



In whatever work I have done I have always aimed 

 at systematic arrangement and uniformity of treatment 

 throughout. But here the immense extent of the subject, 

 the overwhelming mass of detail, and above all the exces- 

 sive diversities in the amount of knowledge of the different 

 classes of animals, rendered it quite impossible to treat all 

 alike. My preliminary studies had already satisfied me that 

 it was quite useless to attempt to found any conclusions on 

 those groups which were comparatively little known, either 

 as regards the proportion of species collected and described, 

 or as regards their systematic classification. It was also 

 clear that as the present distribution of animals is neces- 

 sarily due to their past distribution, the greatest import- 

 ance must be given to those groups whose fossil remains 

 in the more recent strata are the most abundant and the 

 best known. These considerations led me to limit my work 

 in its detailed systematic groundwork, and study of the 

 principles and law of distribution, to the mammalia and 

 birds, and to apply the principles thus arrived at to an 

 explanation of the distribution of other groups, such as 

 reptiles, fresh-water fishes, land and fresh-water shells, 

 and the best-known insect Orders. 



There remained another fundamental point to consider. 

 Geographical distribution in its practical applications and 

 interest, both to students and to the general reader, consists 

 of two distinct divisions, or rather, perhaps, may be looked 

 at from two points of view. In the first of these we divide 

 the earth into regions and sub-regions, study the causes 

 which have led to the difference in their animal produc- 

 tions, give a general account of these, with the amount of 

 resemblance to and difference from other regions; and we 

 may also give lists of the families and genera inhabiting 

 each, with indications as to which are peculiar and which 

 are also found in adjacent regions. This aspect of the 

 study I term zoological geography, and it is that which 



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