130 REPRODUCTION 



flies." With the rapid rate of reproduction in flies, 

 one fly living all summer and producing a hundred 

 or more of eggs, and one-half of her progeny being 

 females, and coming to maturity in two weeks with 

 the same powers, the swarm of flies centering in 

 the city would overtake the fleeing man by sheer 

 increase in number.*^. 



e. Sparrows. This is a bird rather more prolific than 

 is usual. It breeds several times in one season, 

 thus giving it the advantage over birds that nest 

 only once. If we start the season with one pair and 

 assume that they nest three times with four young 

 at each nesting, and live five years, and that one- 

 half are females, we can compute how many young 

 there would be in ten years. 



First year 2 12 young 14 



Second year 14 84 " 98 



Third year 98 588 " 686 



Fourth year 686 4116 " 4802 



Fifth year 4802 28812 " 33614 



And for the remaining five years each pair of these 

 would produce as many as this first pair. 



The student may be interested to find how many 

 there would be in that time. 



f. Man. Man produces much more slowly than any 

 of the forms mentioned. Physically it would be 

 possible for the healthy aboriginal man to reproduce 



'about once in fifteen or eighteen months on an 

 average. Or during the possible reproductive 



•period of about thirty years one pair might produce, 



say, twenty offspring. Before the last one was 

 produced, the oldest would be in its reproductive 

 period, and thus the generations would overlap. As 

 a matter of fact, we do not find families of this size. 

 Marriages take place much later than the beginning 

 of the reproductive period. The lack of strength 



