PRINCIPLES OF ZOOLOGY. 27 



cessive creations, 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 

 knowledge in Zoology. A century ago, the number of 

 known animals did not exceed 8000 ; that is to say, from 

 the whole Animal Kingdom, fewer species were then 

 known than are now contained in many private collections 

 of certain families of insects merely. At the present 

 day, the number of living species which have been satisfac- 

 torily made out and described, is more than 50,000.* The 

 fossils already described exceed 6000 species ; and if we 



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

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

 ber 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 number about the same as the Mammals, 1500 described 

 species, and they 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 

 10,000. There are collections of marine shells, bivalve and univalve, which 

 amount to 5 or 6000 ; and collections of land and fluviatile shells, which 

 count as many as 2000. The total number of mollusks would, therefore, 

 probably exceed 15,000 species. 



Among the articulated animals it is difficult to estimate the number of 

 species. There are collections of coleopterous insects which number 20 to 

 25,000 species ; and it is quite probable, that by uniting the principal col- 

 lections of insects, 60 or 80,000 species might now be counted;' for the 

 whole department of articulata, comprising the Crustacea, the cirrhipeda, 

 the insects, the red-blooded worms, the intestinal worms, and the infuso- 

 ria so far as they belong to this department, the number would already 

 amount to 100,000 ; and we might safely compute the probable number of 

 species actually existing at double that sum. 



Add to these about 10,000 for radiata, including echini, star-fishes, me- 

 dusae, and polypi, and we have about 250,000 species of living animals ; and 

 supposing the number of fossil species only to equal them, we have, at a 

 very moderate computation, half a million of species. 



