CLASSES OF ANIMALS 333 



then do we think of it as a low animal? The answer is 

 that the horse has wonderful and special organs for par- 

 ticular duties, while the ameba has only one cell for all its 

 functions. In the ameba the cell is entirely independent 

 of other cells; in the mammal each cell has given up part 

 of its independence in order that a multitude of cells may 

 live together. 



When many cells live together in an organism, each 

 cell does not need to carry out all the functions of the 

 organism; for there is a "division of labor" among the 

 cells, just as there is among the workmen of a successful 

 factory. We saw the beginnings of this "division of 

 labor" in the case of the hydra (cf. 334). As we ascend 

 the scale of animal forms, we find the division of labor 

 becoming more complete. Thus, the problem of food 

 digestion is worked out in a more and more excellent way. 

 The circulation of the blood, which begins in the con- 

 traction of some part of the blood tubes of an earthworm, 

 ends in the four-chambered heart of birds and mammals. 

 Respiration is first carried out by all the cells; then by 

 some cells that have thinner walls. In higher creatures 

 these thin walls are greatly folded, so that they shall have 

 a large surface, and the result is the gill structure. In 

 amphibians we see the change from gills to lungs, while in 

 reptiles, birds, and mammals gills are not used at all. 



We are likely to think that the mammals now on the earth have 

 always had their present forms. That this is not true is admirably 

 illustrated by the ease of the horse. The fossils of the first horse show 

 that the animal was of about the size of a fox, and that it had five 

 toes to each foot. But the succession of fossils shows that the first 

 and fifth toes became mere splints, producing a three-toed animal as 



