394 ELExMENTS OF BIOLOGY 



Weismann and of De Vries has deeply influenced modern views of 

 the nature of the evolutionary process. Many have come to look 

 upon the fact of mutations, germinal in character, as the whole 

 source of evolutionary change and of the origin of species. All 

 experimental efforts to show that characters acquired by the indi- 

 vidual animal as a result of his experience or of his environment 

 are transmitted to the offspring have either failed completely, or 

 are open to quite different interpretations. Mutations are constantly 

 occurring in various varieties of plants and animals and conse- 

 quently their occurrence is given great weight as a possible source 

 of new species. Moreover, the nature of the process of synapsis in 

 the germ cells suggests that new combinations of genes are con- 

 stantly being effected. The possibility of hybridization is also taken 

 into account. Experimentally it has been possible to induce cross 

 fertilizations between different genera, the resulting offspring being 

 different from either parent. Occasionally such crossings are known 

 to occur in Nature. But in pursuing this interpretation, that re- 

 combinations of genes and of mutations within the gene are re- 

 sponsible for evolution, great difficulties are encountered. For 

 example, observed mutations are usually, but not invariably, Men- 

 delian recessives, and represent alterations of already existing genes. 

 Recessive characters are not apt to evolve, for they are usually 

 heterozygotic in the following generations, unless, fortuitously, back 

 crosses occur. Moreover, the appearance of new genes is exceptional, 

 if it occurs at all. Other, and equally important, objections to the 

 gene theory of evolution have been advanced. Consequently, this 

 interpretation fails to satisfy some biologists. On the other hand, 

 some geneticists are so thoroughly convinced that heredity can be 

 accounted for only on the basis of the gene theory, that when the 

 doctrine of evolution conflicts with the theory of the gene, they 

 advocate that the whole concept of evolution should be thrown 

 overboard. 



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