2 THE TREND OF THE RACE 
be transmitted to the next generation, naturally hold that man’s 
inherited traits can be modified through experiences with his 
social environment. In the writings of Mr. Herbert Spencer, for 
instance, most of the peculiarly social endowments of human 
beings are explained as due to the cumulative inherited effects of 
the experience of men with their fellows. Human nature through 
such a process came to be moulded into conformity with the needs 
of social life, and in the course of time the adjustment, it was 
supposed, would become more and more nearly complete. 
If, however, as most biologists now believe, acquired characters 
are not transmitted to offspring, the social environment never- 
theless is able to influence human heredity in many ways. It may 
determine to a large extent what kinds of variations survive and 
propagate, and it may also determine, to some degree at least, the 
nature of the heredity variations which arise in the germ plasm. 
Whatever forces have been concerned in the evolution of plant 
and animal life doubtless continue to operate in the human species. 
Much still remains to be learned, however, in regard to the factors 
of evolution in the organic world. The subject is still steeped in 
controversy. Opinion among biologists remains undecided as to 
the potency of natural selection, the Lamarckian factor, ortho- 
genesis, isolation and mutation as causes of evolution. And he 
who would throw the most light on the problems of human 
biological evolution would perhaps labor most effectively by 
directing his attention to the lower organisms where it is possible 
to apply rigidly controlled experimental methods. 
But greatly as problems of human evolution would be illumi- 
nated by a knowledge of the way in which evolution has been 
brought about in organisms below man, there would remain a 
multitude of specifically human evolutionary problems which can 
be solved only by the study of human data. The development of 
civilization has brought mankind under influences which have 
never before come into play. In addition to the natural forces to 
which lower organisms are exposed, man has come to live in a 
social milieu which constitutes a very large part of what may be 
called his effective environment. From this circumstance have 
