THE EVOLUTION OF ANIMAL LIFE 



the same kind of individuals to pass its meshes — to survive. But the sieve may 

 chana^e and mav tlien select new kinds of individuals for survival. Evolution 

 now in one direction, now in another, is therefore perfectly possible. 



It should be reiterated that the theory of natural selection, as set forth 

 bv Darwin, does not attempt to explain the nature of variation and heredity. 

 Selection is viewed as a directive rather than a creative factor in evolution. 

 Also, selection cannot be thought to control or direct the evolution of non- 

 adaptive or non-useful characters, unless these are linked in heredity with 

 characters that are adaptive. 



Present-Day Concepts of the Mechanism 

 of Organic Evolution 



The major outlines of Darwin's theory of natural selection stand as a 

 recognized major contribution in the history of evolutionary thought. The 

 soundness of Darwin's conclusions, resting on innumerable observations 

 patiently fitted together into a logical pattern of interpretation, is the more 

 impressive when it is realized that they were drawn in complete ignorance of 

 the basic mechanisms of heredity, and without the significant data yielded by 

 modern experimental work in the environmental relations of animals and 

 plants. In the years between 1859 and 1900, no biological topic was more 

 widely discussed than that of natural selection; the idea of the struggle for 

 existence was particularly questioned. Although numerous biologists busied 

 themselves collecting information that might be brought to bear on the evolu- 

 tion question, there appear to have been virtually no persistent investigations 

 by experimental methods to establish the validity of Darwin's ideas, and his 

 critics did not present any strong evidence against the theory. With the 

 gradual rise of the science of genetics after 1900 and the later application 

 of its principles to studies of population phenomena, and with the develop- 

 ment of ecology as an experimental science that has demonstrated the com- 

 petitive basis and the reality of the struggle for existence, Darwin's ideas 

 have taken their place as the keystone of present-dav evolutionarv theory. 



A major contribution from the field of genetics has been the understanding 

 that mutations constitute the "random heritable variations" postulated by 

 Darwin, and that the establishment of new sets of inherited characteristics in 

 a population is the basis of evolutionary change. Although the production of 

 mutations is understood in a general way, and the general selective action 

 of the environment can be appreciated, it is difficult to determine precisely 

 when or where a new species is established, in a continuously changing group 

 of organisms. An answer to this question depends, of course, on how we de- 

 fine a species. One particularly dynamic view of this important concept holds 

 that a species is the stage, in a process of continued evolutionary divergence, 

 at which the members of formerly freely interbreeding populations have 



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