132 



ZOOLOGY 



Natural 

 selection 

 compared 

 with selec- 

 tion by man 



Modifica- 

 tions of 

 Darwin's 

 theory 



wholly unfit in the Darwinian sense ; their race does not 

 survive. 



5. The phenomena we have just described can be ob- 

 served at any time ; their existence does not admit of 

 dispute. The question is, what have they to do with 

 evolution ? Is the race altered by the survival of the 

 fittest ? The whole matter turns on the question 

 whether, since the survivors differ from those which 

 perish, the differences will be transmitted to future 

 generations. Darwin took this for granted, and was 

 fortified by the experience of mankind in producing 

 many special varieties of animals and plants through the 

 agency of selection. In one sense, of course, man had 

 not produced these things, he had only chosen them ; 

 but their selection and isolation, and often recombina- 

 tion, had in effect changed the character of the popula- 

 tions. Man had done this, as for instance with the 

 sugar beet, in the course of a few years. Was it not 

 reasonable to suppose that Nature could do the same, 

 given almost unlimited time ? 



6. Since Darwin's day our knowledge of the processes 

 of heredity has greatly increased, and consequently the 

 whole subject has had to be reconsidered. It is no just 

 criticism of Darwin, that he did not introduce into his 

 reasoning facts which were then unknown. First came 

 Weismann, the eminent zoologist of Freiburg in Baden, 

 with his theory of the continuity of the germ plasm. 

 He pointed out that each new generation arose from the 

 special reproductive cells of the one before, and conse- 

 quently the effects of environment on the organism 

 could not be inherited. The only exceptions to this 

 rule would be those in which the germ plasm itself was 

 affected. This theory at first caused surprise, but cases 

 brought forward to show the "inheritance of acquired 



