THE HAZARDS OF THE PAST 
the riot and excesses of animal life far beyond any- 
thing we know of. And our line of descent was tak- 
ing its chances amid it all. The widespread blotting 
out of life at the end of Paleozoic time, and again at 
the end of Mesozoic times, when myriads of forms 
were cut off, probably from some convulsion of na- 
ture or some cosmic catastrophe; and again during 
the ice age, when the camel, the llama, the horse, 
the tapir, the mastodon, the elephant, the giant 
sloth, became extinct in North America — how 
fared it with our ancestor during these terrible ages? 
There is no sure trace of him till late Tertiary times, 
and it is probably not more than two hundred thou- 
sand years ago that he assumed the upright attitude 
and began to use tools. Probably in Europe fifty 
thousand years ago he was living in caves, clothed 
in skins, contending with the cave bear and cave 
lion, using rude stone implements, and hunting the 
hairy mastodon, etc. In Asia the probabilities are 
that he was farther on the road toward the dawn of 
history. 
We may think of our descent in the historic period 
under the image of the stream, though of a stream 
many times delayed and diverted, even many times 
diminished by wars and plagues and famine, but a 
stream with some sort of unity and continuity, since 
man became man. The stream of life is like any 
other stream in this respect. Divert or use up part 
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