TIME AND CHANGE 
Numberless specific forms become extinct, but the 
impulse that begat the form does not die out. Thus, 
all the giant reptiles died out — the dinosaurs, the 
mesosaurs — but the reptilian impulse still sur- 
vives. How many types of invertebrates have per- 
ished! but the invertebrate impulse still goes on. 
How many species of mammals have been cut off! 
yet the mammal impulse has steadily gone forward. 
These things suggest the wave that moves on but 
leaves the water behind. The vertebrate impulse 
began in wormlike forms, in the old Paleozoic seas, 
and stopped not till it culminated in man. This im- 
pulse has left many forms behind it; but has this 
impulse itself ever been endangered? If one looks at 
the matter thus in an abstract instead of a concrete 
way, the problem of our descent becomes easier. 
When we look at the evolution of life on a grand 
scale, nature seems to feel her way, like a blind man, 
groping, hesitating, trying this road and then that. 
In some cases the line of evolution seems to end in a 
cul de sac beyond which no progress is possible. The 
forms thus cornered soon become extinct. The 
mystery, the unaccountable thing, is the appearance 
of new characters. The slow modification or trans- 
formation of an existing character may often be 
traced; natural selection, or the struggle for exist- 
ence, takes it in hand and adapts and perpetuates it, 
or else eliminates it. But the origin of certain new 
parts or characters — that is the secret of the evolu- 
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