610 



COLLEGE ZOOLOGY 



swimming, and the bills for capturing in- 

 sects, crushing seeds, or tearing flesh (Fig. 

 316). Such external adaptations are abun- 

 dantly exhibited by all animals. They must 

 fit a species for life in its environment, or 

 that species cannot survive in the struggle 

 for existence that is continually taking place. 

 They involve all those activities necessary 

 for the individual to maintain itself and to 

 reproduce others of its kind. 



Internal adaptations are concerned with 

 the relations of the organs of the body to 

 one another and with their physiologic ac- 

 tivities. If these organs do not cooperate 

 successfully in the manifold processes in- 

 volved in locomotion, digestion, circulation, 

 respiration, etc., the animal dies. How often 

 individuals are rendered unfit by their in- 

 ternal adaptations is indicated by the large 

 number of deaths that occur among people 

 of the white race when exposed to tropical 

 diseases, such as malaria and yellow fever. 

 The principle of organic evolution and the 

 evidence in favor of that principle depend 

 on the assumption that the degree of 

 adaptability to the environment determines 

 whether an animal shall live or die in the 

 struggle for existence. 



Adaptive radiation and 

 adaptive convergence 



The methods of locomotion in mammals 

 may be used to illustrate divergent adapta- 

 tions in relation to various types of environ- 

 ment. Because of the competition for liv- 

 ing space and food, there is a tendency for 

 each group of organisms to spread out and 

 utilize as many different habitats as possi- 

 ble. This evolution, from a single ancestral 

 species to a variety of forms which occupy 

 different habitats is called adaptive radia- 

 tion or divergent evolution. Figare 431 il- 

 lustrates that from a primative mammal, 

 different forms have evolved that are 

 adapted to various habitats. The specializa- 

 tions or adaptations indicated are for run- 

 ning, digging, swimming, hanging, flying, 



and jumping. The great range of adapta- 

 bility of the mammals has made it possible 

 for members of this class to compete suc- 

 cessfully with other types of animals on the 

 surface of the earth, under the ground, in 

 trees, and in the water, and to become the 

 dominant animals in the long course of 

 evolution. 



Adaptive convergence or convergent 

 evolution, which is the converse of adaptive 

 radiation, may be illustrated (Fig. 432) by 

 the streamlined body of a fish (shark), an 

 extinct reptile (ichthyosaur), and a mam- 

 mal (porpoise). Why should reptiles and 

 mammals that live in water superficially 

 resemble a fish in form? Natural selection 

 has favored in aquatic animals, whether 

 they be fish or mammals, those characteris- 

 tics which fit them for life in water. As far 

 as body form is concerned, the ancestors of 

 a fish and a porpoise were probably much 

 less similar than these animals are today. 

 Note that the convergence has affected only 

 some traits which are of special importance 

 in adapting the mammal to life in water. 

 Many body structures, such as the lungs, 

 have not converged. 



EVIDENCE IN FAVOR 

 OF ORGANIC EVOLUTION 



Practically all biologists believe in the 

 evolution of plant and animal life, that is, in 

 the principle that species of plants and ani- 

 mals now living have evolved from other 

 species. The evidence for organic evolution 

 is compelling. It is a well-established fact 

 that all protoplasm is irritable; therefore, it 

 should no longer be regarded as only a 

 theory. One can hardly argue about the 

 existence of established facts, one can only 

 be ignorant of them. However, there are 

 diflferences of opinion among biologists as 

 to how evolution has taken place and as to 

 its causes. The principle of evolution is 

 based on a number of different types of 

 evidence, some of which are presented 

 briefly in the following paragraphs. 



