GENERAL ZOOLOGY 



that man has undergone an evolutionary process comparable with those of 

 other forms of life. 



Theories Concerning Organic Evolution 



Organic evolution as an historic fact is attested by the evidence outlined 

 in the preceding section. We may now examine the more important theories 

 concerning the factors that have conditoned organic evolution. Most notable 

 among these are Lamarck's Theory of the Inheritance of Acquired Character- 

 istics and Darwin's Theory of Natural Selection. We shall also discuss 

 briefly conclusions from recent studies in genetics and ecology. Any compre- 

 hensive theory must take into account both internal and external factors. 

 Among internal factors are the phenomena of heredity, variation, reproduc- 

 tion, and development; external factors include all environmental conditions 

 that affect individuals and populations. 



Lamarck's Theory of the Inheritance of Acquired Characteris- 

 tics: Historical. The works of Lamarck (1744-1829), written principally 

 during the first quarter of the nineteenth century, postulated that evolu- 

 tionary changes are conditioned by the inheritance of characters acquired 

 during the life of the individual. Lamarck built this theory upon the earlier 

 works of another Frenchman, Buffon (1707-1788), who had previously stated 

 the concept of evolution as opposed to special creation. Many of Lamarck's 

 statements, examples of which will be given later, appear fanciful in the light 

 of modern knowledge. His essential claim that characters acquired by an 

 individual during its lifetime are inherited by its off'spring, and thus produce 

 evolutionary changes, has never been substantiated. Nevertheless, the pub- 

 lication of Lamarck's theory served to focus attention on the subject of organic 

 evolution. 



Explanation of Lamarck's Theory, In its modern form the Lamarckian 

 theory states that during the life of an individual new characters can be ac- 

 quired by use or disuse of parts, and also by the direct eflfects of the environ- 

 ment on somatic cells. It is a familiar fact that the use of muscles increases 

 their development; and the old adage "practice makes perfect" finds many 

 illustrations in the refinement of neuromuscular coordination through pro- 

 longed repetition of actions. Conversely, disuse leads to deficiency or even 

 complete loss of function, as illustrated by the fanatic of India who holds 

 a limb in one position until it cannot be moved, and by many other examples. 

 Such effects of use and disuse, and many effects of the environment on the 

 individual, are known technically as acquired characters. There can be no 

 doubt that they occur; the Lamarckian theory holds, however, that such 

 effects constitute heritable variations and thus condition evolution. 



The process supposed to occur in nature may be illustrated by citing some 

 of Lamarck's own examples, in somewhat modified terms. If swift-footed 

 animals, such as deer, have acquired their ffeetness by running from their 



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