192 THE INDIVIDUAL ORGANISM 



Concentrated nervous system of the vertebrates. Concentration of 

 nervous tissue has gone much farther in the vertebrates (Fig. 13.8) than 

 in any other group of animals. Within this group we note an increasing 

 degree of complexity in the structure and function of the nervous system, 

 in general running parallel to complexity of body organization and cul- 

 minating in the very highly concentrated and strongly cephalized nervous 

 system of man. 



Summing up our survey of form and function in the animal kingdom, 

 we see that no matter what animal we choose to examine, it proves to 

 have the same essential needs and presents the same problems. Every 

 individual animal has to obtain organic food (proteins, carbohydrates, 

 and fats), to prepare this food for assimilation, to utilize part of it for 

 maintaining its own protoplasm and part as fuel for the production of 

 energy. Every animal requires oxygen for internal combustion and must 

 eliminate the products of protein, fat, and carbohydrate catabolism. 

 Every animal shows irritability and appropriate responsiveness to ex- 

 ternal and internal stimuli. And finally, every animal must have the 

 ability to produce other animals like itself. 



In no fundamental respect, therefore, do the other patterns of animal 

 life differ from that typified by man so far as the basic problems of main- 

 tenance are concerned. The marked differences in the types of structural 

 organization that are capable of carrying on the functions listed above are 

 due almost wholly to different patterns and degrees of cell, tissue, and 

 organ differentiation and the resulting degrees of "division of labor." 

 Generally speaking, the higher the degree of such differentiation the 

 greater the efficiency of the organism and the greater its freedom from 

 narrow limitations as to where it can maintain itself in nature. 



THE MAJOR PATTERNS OF PLANT LIFE 



We have just seen that among animals the tasks of individual main- 

 tenance may be accomplished by a variety of types of organization, 

 and the same is true of plants. However, all typical plants possess chloro- 

 phyll, and manufacture food by photosynthesis. This fact accounts not 

 only for all the major features of their organization but also for the 

 absence of many of the features characteristic of the animal. As we have 

 previously pointed out, the plant has no need of nervous system, special 

 sense organs, or locomotor organs, for it does not have to seek its food 

 and is forced by its method of nutrition to remain fixed in one spot. It 

 requires neither digestive system nor excretory system, for its food is 

 made within its cells, and its wastes are for the most part either reutilized, 

 stored in tissues, or diffused into the atmosphere from the leaves. Its 

 problems are, in fact, far simpler than those of the animal. This not only 

 explains why even the highest plants are far less complex than the higher 



