INTRODUCTION 
NATURALISTS have come to believe that all of the higher 
animals and plants have descended from simpler forms which 
lived in the past; and-that these in turn were derived from even 
simpler ancestors. Indeed, it is possible that all animate nature 
is the offspring of one primitive living cell which contained within 
itself the power of giving rise to all of the plant and animal life 
of our world. 
But although we know not how or when life originated, science 
has been able to make known some few of the remarkable changes 
which have come over animate forms under the influences of evolu- 
tion, and natural selection. 
Excellent examples of such changes are exhibited in the great 
Hall of Vertebrate Fossils at the American Museum of Natural 
History, where we may see a series of fossil skeletons which prove 
that the horse was once a four-toed creature hardly larger than a 
fox but that now it walks upon its middle toe, the side ones having 
disappeared. Another series of fossils shows that in Eocene times 
the camel was also a little four-toed creature; but now there are but 
two toes on each foot, the side ones having disappeared. 
A careful study of living creatures has shown that, while off- 
spring usually bear a close resemblance to their parents, a few 
depart widely from the parental types, and that some of these de- 
partures show a strong tendency to maintain themselves, through 
inheritance, for generation after generation. But this is not all, 
for we know that animals and plants tend to increase at a rate so 
enormous that, should all survive, the land would soon be densely 
covered and the ocean completely filled with living creatures. 
This, however, is prevented by the constant competition for life. 
Only those few that are able to conquer in the strife for food and 
Space can survive, and myriads of the weak and unfit must perish. 
Whole races have succumbed to this competition. Not one of the 
