364 NATURAL HISTORY. 
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 Moilusks. 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 compute the whole 
number at 200,000. If we add to the above 10,000 for 
the Radiates, we shall have about 250,000 species. It is 
also estimated by Agassiz that there is about the same 
number of species of fossil animals; that is, those which 
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 Iguanodon (§ 326), and 
the Ammonites (§ 551). 
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- 
malculz, 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 
58,000 in a cubie 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 
