2 SPHEEE AST) 



and in order to know what end he had in view, we must have 

 regard to his previous labours, and to the circumstances under 

 which the work was executed. Without this, although we 

 may perhaps enjoy the perfection of the whole, and admire 

 the beauty of its details, yet the spirit which pervades it will 

 escape us, and many passages may even remain unintelligible. 



5. So, in the study of Nature, we may be astonished at 

 the infinite variety of her products, and may even study some 

 portion of her works with enthusiasm, and nevertheless re- 

 main strangers to the spirit of the whole, ignorant of the plan 

 on which it is based ; and may fail to acquire a proper con- 

 ception of the varied affinities which combine beings together, 

 so as to make of them that vast picture, in which each animal, 

 each plant, each group, each class, has its place, and from 

 which nothing could be removed without destroying the proper 

 meaning of the whole. 



6. Besides the beings which inhabit the earth at the pre- 

 sent time, this picture also embraces the extinct races which 

 are now known to us by their fossil remains only. These are 

 of very great importance, since they furnish us with the means 

 of ascertaining the changes and modifications which the Ani- 

 mal Kingdom has undergone in the successive creations which 

 have taken place since the first appearance of living beings. 



7. It is but a short time since it was not difficult for a 

 man to possess himself of the whole domain of positive know- 

 ledge in Zoology. A century ago, the number of known 

 animals did not exceed 8000 ; that is to say, in the whole 

 Animal Kingdom, fewer species were then known than are 

 now contained in many private collections of certain families 

 of insects alone. At the present day, the number of living 

 species which have been satisfactorily made out and described, 

 is more than 50,000.* The fossils already described exceed 



* The number of vertebrate animals may be estimated at 20,000. 

 About 1500 species of mammals are pretty precisely known, and the 

 number may probably be carried to about 2000. 



The number of Birds well known is 4 or 5000 species, and the probable 

 number is 6000. 



The Reptiles, like the Mammals, number about 1500 described species, 

 and will probably reach the number of 2000. 



The Fishes are more numerous ; there are from 5 to 6000 species in the 

 museums of Europe, and the number may probably amount to 8 or 10,000. 



The number of Mollusks already in collections," probably reaches 8 or 



