68 ADAPTATION AND PROGRESS 



good reason has he been called the liberator of the human mind 

 and spirit. Patient, long-continued investigation to discover the 

 cause and laws of variation is now the sine qua non of success in 

 every science. His catholic spirit, generous appreciation of the 

 discoveries of others together with a humble estimate of his own 

 merit form a rare combination in one who is generally estimated 

 as the most influential thinker of the nineteenth century. The 

 five links in his chain of causes leading to the origin of species 

 stand today, though some have been interpreted differently, and 

 causes minimized by him have been raised to rank with that of his 

 great theory. Especially is this true concerning the factor of 

 geographical isolation. 



The fittest to survive, in Darwin's thought, are those best 

 adapted to their environment. Spontaneous variations of use in 

 the struggle for existence have been preserved and transmitted by 

 heredity while variations disuseful have been eliminated. Not 

 only is adaptation emphasized by him but adaptability, that is, 

 power in the organism to adapt itself to a changing environment. 

 We have brought to our attention also the fact that the variations 

 need not always be useful to the individual providing they are 

 useful to the species in its contest with other species. Connection 

 is made between passive and active adaptation and the principle 

 of struggle and survival applied to the development of the higher 

 human faculties and the evolution of races. Natural selection 

 is supplemented by sexual selection to account for secondary 

 sexual differences. 



We must pass now to the contributions of some other biologists 

 who have supplemented and corrected the work of their master. 



August Weismann (1834- ) 

 Continuity of the Germ Plasm 



August Weismann the " Sage of Freiburg '' is especially worthy 

 of consideration in our discussion as his investigations and teach- 

 ings mark a turning-point in biological and to a certain extent in 

 sociological theory, for the doctrine of natural selection was 

 somewhat on the wane when he began to write but with him it 



