INTRODUCTION 7 



although, as Darwin himself said, not the sole agency by 

 which this is brought about, is natural selection. 



It should not, however, be forgotten that the great 

 principle enunciated by Darwin which laid the foundation 

 of the modern science of evolution was descent with modifica- 

 tion. Darwin's theory is too often spoken of as the theory 

 of natural selection, but this was only an explanation of the 

 method by which he considered the change of one species 

 into another was chiefly brought about. As stated by 

 De Vries, * The theory of descent remains unshaken even 

 if our conception concerning the mode of descent prove 

 to be in need of revision ' (8). 



The law of natural selection is founded on the fact that 

 variations arise among groups of organisms, and those 

 variations which are of advantage to the organism in the 

 struggle for existence are perpetuated by heredity. Darwin 

 states in the Origin of Species (3 a), ' natural selection acts 

 solely by accumulating slight, successive, favourable varia- 

 tions, it can produce no great and sudden modifications/ 

 but Darwin considered that natural selection, while it has 

 been the principal agent, has not been the sole influence in 

 the evolution of species ; he considered it was aided in an 

 important manner by the inherited effect of the use and 

 disuse of parts. 



The discoveries of Mendel (5) have, however, of late years 

 considerably modified several of Darwin's conclusions. 

 These important discoveries were for a long time passed 

 over in silence, but have lately obtained great prominence 

 among evolutionists, and they explain much in the doctrine 

 of heredity which has hitherto been obscure. Mendel's 

 researches -were repeated and confirmed by De Vries, 

 Torrens, and Tschermak, and published by them in 1900. 



Mendel's first series of experiments were made on the 

 edible pea. A variety of this pea with tall stems was crossed 

 with a short-stemmed variety. The next generation, F r 

 showed all long-stemmed plants, and this tallness he described 

 as the ' Dominant ' character. These tall plants, when self- 

 fertilized, gave in the next generation, F 2 , both tall and short 

 plants in a definite numerical relation to one another, this 

 being in the ratio of, approximately, three tall to one short. 



