THE STUDY OF ZOOLOGY 23 



kingdom, each stem in turn sending out branches that represent 

 the subdivisions of plants and animals (Fig. 6). Such a tree 

 would show the relationships between organisms in a very exact 

 manner if knowledge of the evolutionary processes were complete, 

 but since this knowledge is at its best rather fragmentary, 

 the construction of the tree of life is a difficult matter and subject 

 to continual alteration. 



Descent. — When it is held that the higher animals are derived 

 or descended from animals simpler in form and organization, it 

 is not meant that the lower animals living today are the ancestors 

 of the higher ones. Thus, it would be incorrect to speak of 

 monkeys or apes as the ancestors of Man, although it is true 

 that Man is more closely related to them than to any other 

 animals. The fact is that Man and monkey probably had some 

 common ancestor from which monkeys were evolved on one hand 

 and Man on the other — the common ancestor having in the mean- 

 time disappeared from the living fauna. Since the same princi- 

 ple applies to other groups of animals and plants, it follows that 

 the part of the genealogical tree that represents the living popula- 

 tion of the earth includes only the twigs — the trunk and its 

 branches, representing the connecting links between living forms, 

 having dropped out and become extinct. 



Natural Affinities. — Evidence of relationship between organ- 

 isms may be obtained from a study of living animals and plants. 

 The great Swedish naturalist, Linnaeus (1707-1778), who 

 founded the modern system of classification, was not an evolu- 

 tionist, but he was able nevertheless to classify animals and 

 plants with a remarkable degree of accuracy by carefully noting 

 their resemblances and differences. The point is, however, that 

 these resemblances, or natural affinities, as Darwin called them, 

 find their rational explanation in the theory of evolution, with the 

 result that the application of the evolution principle has had a 

 clarifying effect on the problems of classification. 



Species. — In the modern scheme of classification the species 

 constitutes the basic group of individuals. A species may be 

 simply defined as the offspring of similar parents. The members 

 of similar species resemble one another because they are 

 descended from common ancestors. Species have become more 

 or less sharply differentiated from all other species by the dis- 

 appearance of intermediate forms. Owing to variation, the 



