364 NATURAL HISTOEY. 



of species of Vertebrates is probably 20,000, of which 

 the Mammals are 2000, the Birds 6000, the Reptiles 2000, 

 and the Fishes 8000 or 10,000. There are probably over 

 15,000 Mollusks. The Insects are the most numerous 

 class of animals, there being already collected from 60 

 to 80,000 species. Of all the Articulates there are about 

 100,000 now known, and it is safe to comjDute the whole 

 number at 200,000. If we add to the above 10,000 for 

 the Radiates, we shall have about 250,000 sjjecies. It is 

 also estimated by Agassiz that there is about the same 

 number of species of fossil animals ; that is, those vrhich 

 are not now in existence, but which are known to have 

 existed by the remains that we find of them in the rocks 

 and in the earth. I have noticed a few of these in pass- 

 ing, as the Mastodon (§ 139), the Iguanpdon (§ 326), and 

 the Ammonites (§ 551). 



f 635. But farther than all this, we can get no adequate 

 idea of the abundance of animal life if we do not take 

 into view the minuter living forms, as well as those 

 which are ordinarily noticed. These I have not consid- 

 ered, because it would lead me into too wide a field. 

 Quite large portions of the earth — of its rocks, and mount- 

 ains, and sand, and mud, and dust — are made up in part 

 of the remains of minute animals, called, therefore, ani- 

 malculae, or, in English, animalcules. Some of these are 

 so small that their structure can not be made out except 

 by the aid of the microscope, and some can not even be 

 seen at all by the naked eye. For example, the stone 

 used for building in Paris, and in all the country round 

 it, is so full of the shells of an animalcule, that there are 

 58,009 in a cubic inch, or three thousand millions in a 

 cubic yard. This animal belongs to a group which are 

 called Foraminifera, because their shells are full of little 

 foramens or openings. The substance within the cham- 

 bers of the shell is mostly a translucent jelly, and through 

 the openings branch out root-like legs, on which it is cu- 

 rious to see the animal walk. Foraminifera, perhaps of 



