CHAPTER 21 



THE CAUSES OF EVOLUTION 



Evolution as the historic fact is attested by the evidence outhned 

 in the preceding chapter. It is impossible to present this evidence 

 of the fact without indicating what is known of the course of evolu- 

 tion, since fact and course are so intimately related. It is, how- 

 ever, possible to know much concerning the fact and course of 

 organic evolution without understanding its causes. Such is the 

 position of biological science at the present day. Evolution is by 

 far the most reasonable explanation of the history of animal hfe, 

 but the course of evolution is known only in a general way and its 

 causes are still uncertain. Attempts to discover and formulate 

 these causes are represented by the Lamarckian Theory of the 

 Inheritance of Acquired Characteristics, the Darwinian Theory of 

 Natural Selection, the Mutation Theory of de Vries, various the- 

 ories of Orthogenesis, and the Theory of Evolution by Hybridiza- 

 tion which has been suggested by the Mendehan laws of heredity; 

 together with lesser theories, hke Darwin's Theory of Sexual 

 Selection and the Theory of Isolation, which may be classified under 

 those enumerated. These theories of evolutionary causation are 

 the subject matter of the present chapter. Evolution depends 

 primarily upon variations by which new characters make their 

 appearance, and upon heredity, by which variations are passed 

 on to subsequent generations. Any theory of evolutionary causa- 

 tion that is complete must deal with these two phenomena. 



The Lamarckian Theory of the Inheritance of Acquired 



Characteristics 



Historical. — A theory of the evolutionary process as a whole and 

 of the inheritance of acquired characteristics as the cause of evolu- 

 tion was formulated in the works of Lamarck (1744-1829), written 

 principally during the first quarter of the nineteenth century. 

 Another Frenchman, Buff on (1707-1788), had previously formu- 



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