DIFFERENCES BETWEEN ANIMALS AND PLANTS. 37 



The organs of the body are composed mainly of these tissues, and 

 present the greatest diversity of structure and function ; but they may 

 be roughly arranged, according to their functions, into four groups : 

 organs of nutrition, such as the digestive, circulatory, and respiratory 

 organs ; organs of reproduction, organs of motion, and organs of rela- 

 tion, such as the nervous system and sense-organs, and organs of de- 

 fense or protection, such as horns or spurs. Except among the lower 

 and simpler groups the individuals are not organically united, as in 

 plants, into a community, although such communities as pairs, or flocks, 

 or herds, are frequent. In such a community as a hive of bees the dif- 

 ferent individuals are specialized for the good of the whole, and are 

 unable to exist apart ; and the community is as real as in the case of a 

 plant, although the connection is not material, but purely ideal. 



Physiologically, animals are characterized by the fact that, with few 

 exceptions, they are able to receive solid food into a definite internal 

 digestive cavity, in which it is digested, and then absorbed through the 

 wall of the cavity. They absorb oxygen from the surrounding medium, 

 air or water, and, in addition to certain inorganic substances, take into 

 their bodies, as their proper nutritive material, complicated protein 

 compounds, which they derive either directly or indirectly from plants. 

 Through the oxidation of these compounds they form substances of a 

 simpler chemical structure, such as carbonic acid, water, and ammonia, 

 and discharge these through their bodies as waste. Since the sum of the 

 chemical changes which take place in the animal is the breaking down 

 of the highly-complex protein-molecules, derived from plants, into the 

 simpler molecules of water, ammonia, and carbonic acid, it results that 

 the potential energy thus set free is shown by the animal as " vital 

 force," and the body of the animal is therefore a magazine or store- 

 house of force which may manifest itself as animal heat or light, or as 

 sensible motion, or nervous disturbance ; an animal, then, is an organ- 

 ism which has the power to change the potential energy of vegetable 

 protein by oxidation into " vital force," which may manifest itself as 

 animal heat, or, in the case of many marine animals, as light, or by 

 peculiar disturbances of the nerves and muscles, organs which are pecul- 

 iarly diagnostic of animals. The changes of the muscles result in mo- 

 tion, either of the animal as a whole or of the various parts in relation 

 to each other. The structure and functions of the nervous system of 

 one of the higher animals are so entirely different from any other phe- 

 nomena, that they seem to be sui generis and peculiar ; but we must 

 not forget that there are true animals which entirely lack a nervous 

 organization, and that in the history of each individual, as well as in the 

 history of the animal kingdom, we may pass without any considerable 

 break from animals with a complicated system of nerves and sense- 

 organs to animals which give no evidence of conscious life, and are no 

 more sensitive than ordinary plants. The nervous system of an animal 

 may be roughly described as a regulative apparatus by which the vari- 



