PEIXCIPLES OF ZOOLOGY. 3 



6000 species ; and if we consider that wherever any one stra- 

 tum of the earth has been well explored, the number of spe- 

 cies discovered has not fallen below that of the living species 

 which now inhabit any particular locality of equal extent, and 

 then bear in niind that there is a great number of geological 

 strata, we may anticipate the day when the ascertained fossil 

 species will fa*r exceed the living species.* 



8. These numbers, far from discouraging, should, on the 

 contrary, encourage those who study Natural History. Each 

 new species is, in some respects, a radiating point which throws 

 additional light on all around it ; so that as the picture is en- 

 larged, it at the same time becomes more intelligible to those 

 who are competent to seize its prominent traits. 



9. To give a detailed account of each and all of these 

 animals, and to show their relations to each other, is the task 

 of the Naturalist. The number and extent of the volumes 

 already published upon the various departments of Natural 

 History show, that only a mere outline of so vast a domain 

 could be given in an elementary work like the present, and 

 that none but those who make it their special study can be 

 expected to survey its individual parts. 



10,000. There are collections of marine shells, hivalve 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 there- 

 fore 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 

 collections 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, as 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, echini, star-fishes, medusae, and 

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

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

 moderate computation, half a million of species. 



* In a separate work, entitled " Nomenclator Zoologicus" by L. Agas- 

 siz, the principles of nomenclature are discussed, and a list of the names 

 of genera and families proposed by authors is given. To this work those 

 are referred who may desire to become more familiar with nomenclature, 

 and to know in detail the genera and families in each class of the Animal 

 Kingdom. 



B 2 



