8 Professor Gregg tVilson on 



under the influence of environment — developed new habits and 

 consequent modifications. We shall find, when we come to 

 consider more recent views of evolution, that this belief in the 

 striving of the organism in response to environmental stimulus 

 has been revived. 



A second epoch in the history of modern thought on 

 evolution begins with Charles Darwin, whose work on the Origin 

 of Species " appeared in 1859. He may be said to have converted 

 the world to belief in the fact of evolution. In his explanation 

 of the change of species, he accepted the Lamarckian doctrines, 

 but he added the idea of the surviA'al of the fittest or most 

 favourable variations in the struggle for existence. He regarded 

 the slight differences that a])pear among members of a family as 

 the determiners of life or death ; and he dwelt on the fact that a 

 variation that secured survival of an individual at the same time 

 provided a chance that there would be off'spring of that individual, 

 with its useful characteristics. He changed the point of view of 

 the scientific world, for whereas Lamarck had laid stress on 

 modification of the body of the adult animal, Charles Darwin 

 emphasized the importance of variations in the egg. He regarded 

 these congenital variations as heritable, and he showed how 

 useful modifications might be built up by successive additions, 

 generation after generation. 



Weismann (1834 — 1914) out-Darwined Darwin. He pro- 

 claimed the "all-sufficiency of natural selection "* and discredited 

 Lamarckism. He maintained that there was no proof of the 

 transmission of acquired characters, and that owing to the 

 isolation of the germ-plasm such transmission was inconceivable. 

 His work on the nature of the germ-plasm was most important.! 

 He strove to show that hereditary characters are represented by 

 definite determinants " in the chromosomes of the germ 

 nucleus, and that development of the egg is essentially an 



* Contemporary Review, September and October, 1893. 

 i " The Germ Plasm : a Theory of Heredity." 1893, 



