THE EPIC OF EVOLUTION 517 



or desire on the part of the organism to meet new conditions. The 

 changes wrought by these means during the lifetime of the individual, 

 he postulated, were then handed on through heredity to following 

 generations. The latter assumption turned out to be a weak link 

 in the chain. 



There is plenty of evidence that environment, directly in the case 

 of plants and indirectly through the nervous system in animals, 

 does cause modification in the structure and behavior of animals 

 and plants. Arctic animals, for example, develop a thick pelt, and 

 wind-swept trees grow in a leaning attitude in accordance with 

 prevailing winds. There is no doubt, either, of the truth of his second 

 assumption. Use does increase, and disuse decrease, the develop- 

 ment of muscles, as every athlete knows. The lungs of opera singers 

 become enlarged, while unused organs in general tend to diminish. 



The third postulate in his theory is not so obvious, and Lamarck 

 himself did not stress it. According to this idea, which Lamarck's 

 opponents made ridiculous, the desire of the ancestral deer, for 

 example, to browse on leaves higher up off the ground caused it to 

 stretch its neck until, after some generations of stretching, it became 

 a giraffe. It goes without saying that plants, lacking the mechanism 

 of a nervous system through which to express "desires," are excluded 

 from this method of attaining evolutionary ends. 



There is no doubt, however, that certain changes during the lifetime 

 of the individual are everywhere brought about by environmental 

 influences and the effects of use and disuse. All such evidence would 

 offer an obvious explanation of the method of evolution if only there 

 was assurance that during the lifetime of the individual acquisitions 

 are passed on. Lamarck did not question that this was so, nor did his 

 contemporaries. It remained for August Weismann (1834-1914) to 

 point out many years later the improbability of the "inheritance of 

 acquired characters." The great service of Lamarck was to over- 

 throw the old idea of the fixity of species, and to suggest a reason- 

 able hypothesis concerning the origin of variations, which must be 

 the point of departure for every theory of evolution. 



Following Lamarck's work there were three avenues open to the 

 seeker after truth about evolution : (a) to retain belief with Linnaeus 

 and Cuvier in the fixity of species, with no evolutionary transforma- 

 tion ; (6) to accept Lamarck's theory of the causes of variation and 

 the inheritance of acquired characteristics ; or (c) to find some other 

 explanation of how evolution came about. 



