GENERAL BIOLOGY. 29 



tries to perform every office for himself, he is at once carpenter, 

 blacksmith, shoemaker, and much more, with the natural re- 

 sult that he is not efficient in any one direction. A community 

 may be judged in regard to its degree of advancement by the 

 amount of division of labor existing within it. Thus is it with 

 the animal body. We find in such a creature as the fresh-water 

 Hydra, consisting of two layers of cells forming a simple sac, a 

 slight amount of advancement on Amoeba. Its external surface 

 no longer serves for inclosure of food, but it has the simplest 

 form of mouth and tentacles. Each of the cells of the internal 

 layer seems to act as a somewhat improved or specialized Amoe- 

 ba, while in those of the outer layer we mark a beginning of 

 those functions which taken collectively give the higher ani- 

 mals information of the surrounding world. 



Looking to the existing state of things in the universe, it is 

 plain that an animal to attain to high ends must have powers 

 of rapid locomotion, capacity to perceive what makes for its in- 

 terest, and ability to utilize means to obtain this when perceived. 

 These considerations demand that an animal high in the scale 

 of being should be provided with limbs sufficiently rigid to sup- 

 port its weight, moved by strong muscles, which miist act in 

 harmony. But this implies abundance of nutriment duly pre- 

 pared and regularly conveyed to the bones and muscles. All 

 this would be useless unless there was a controlling and ener- 

 gizing systemi capable both of being impressed and originating 

 impressions. Such is found in the nerves and nerve-centers. 

 Again, in order that this mechanism be kept in good running 

 order, the waste of its own metabolism, which chokes and poi- 

 sons, must be got rid of— hence the need of excretory apparatus. 

 In order that the nervous system may get sufficient informa- 

 tion of the world around, the surface of the body must be pro- 

 vided with special message-receiving offices in the form of 

 modified nerve-endings. In short, it is seen that an animal as 

 high in the scale as a mammal must have muscular, osseous 

 (and connective), digestive, circulatory, excretory, and nervous 

 tissues ; and to these may be added certain forms of protective 

 tissues, as hair, nails, etc. 



Assuming that the student has at least some general knowl- 

 edge of the structure of these various tissues, we propose to tell 

 in a simple way the whole physiological story in brief. 



The blood is the source of all the nourishment of the organ- 

 ism, including its oxygen supply, and is carried to every part of 



