DIFFICULTIES IX CONSTRUCTION OF NATURAL SYSTEM. 47 



associate the different groups of animals from their correspondence 

 in some one or two simple and easily-recognised characters all 

 the rest being neglected. By following out such a principle, we 

 should not only class Whales with Fishes, because they inhabit the 

 water, and have their bodies formed for swimming, but we should 

 also place the Bats among Birds, because they fly, and group to- 

 gether a great many Reptiles and ordinary Mammals, because they 

 walk upon four feet. No such classification could be of the least 

 use ; and, consequently, all artificial systems have been long given 

 up in the arrangement of the Animal Kingdom, Zoologists 

 applying themselves to the completion of a Natural Classification, 

 in which every animal may be placed among those to which it 

 bears the greatest general resemblance, and in which its relation? 

 to others may be exhibited. 



27. There are great difficulties attending the completion of 

 such a System, however, arising from various causes. In the 

 first place, of the whole number of animals at present existing on 

 the surface of the earth, a large proportion are probably as yet 

 altogether unknown to us. This proportion is less, however, 

 in some tribes than in others. Of the larger Mammals or 

 Reptiles, for instance, it is not probable that many living species 

 remain to be discovered ; yet, of the smaller species, it is certain, 

 that a large number must be residents of countries which have 

 been as yet but imperfectly explored. This is still more the case in 

 regard to Birds ; and of the whole number of species of Insects, 

 which people our globe, we may safely affirm, that but a small 

 proportion have been yet collected by Entomologists, although 

 the amount of those at present contained in their cabinets pro- 

 bably far exceeds 100,000. The reasons are obvious why so 

 little, comparatively speaking, has yet been done for this depart 

 ment of Zoology, notwithstanding the number of persons engaged 

 in the pursuit of it ; for the small size of the greater number of 

 the objects it embraces, causes them to escape the notice of ordi- 

 nary observers ; added to which, there are many species that 

 only exist in their perfect state during a few days or even hours, 

 and are consequently very liable to elude the vigilance of the 

 most diligent Entomologist. Of the animals which inhabit the 



