If we represent the time during which it has been possible for animal life to exist upon our planet by a line of this length, then the tiny line just below indicates the age during which man (or a creature more or less resembling man) has lived upon this earth. Man was the last to come but the first to use his brain for the purpose of conquering the forces of nature. That is the reason why we are going to study him, rather than cats or dogs or horses or any of the other animals, who, all in their own way, have a very interesting historical development behind them.
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