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. 
Aucust 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 
