its organs. It is economy therefore for tHem to do their 

 growing during the larval stages when they are simpler, 

 and moulting is more easy. During this time they store 

 up food enough to provide for the metamorphosis, and the 

 making of all the adult parts. It is possible therefore to 

 see that the metamorphosis may help make reproduction 

 and individual development more successful. It makes the 

 species rather more adjustable to the conditions of life. 



CHAPTER TWENTY-FOUR. 

 THE RATE OF REPRODUCTION. 



1. How Fast Must Organisms Propagate? Reproduction 

 is to keep the species going. To accomplish this, as we 

 have seen in former chapters, each pair must during their 

 life bring to maturity at least two individuals. If they 

 succeed in doing more than this the species increases in 

 numbers; if they do less the species diminishes. On the 

 average this is what each species does. Some may be 

 gaining, but if they do, it is probably at the expense of 

 other species. If a certain species of birds temporarily 

 increases in a locality, the insects on which they feed will 

 grow fewer there. On the other hand, if the birds 

 should decrease the insects are likely to increase. The 

 sum of life on the earth probably does not increase or 

 decrease greatly from generation to generation. Man, at 

 present, is increasing in numbers; and so, perhaps, are 

 those plants .and animals that he cultivates for his uses. 

 But the forest trees, the wild plants, the song and game 

 birds, and the larger wild animals are decreasing with his 

 progress. Thus the total life remains about the same. 



2. What Determines the Rate of Reproduction? How 

 many offspring a plant or animal must produce that two 

 may come to maturity will depend on the favorable or 

 unfavorable conditions through which the young must go 

 to reach maturity. Among these ' are the climatic 

 conditions which favor or endanger the young, the amount 

 of the food supply and the ease of getting it, and the 

 enemies that prey upon the young. But most of all it 

 depends on the protection and care which the parents 

 give to their eggs and their young. How many must 

 be produced depends on such things as those just 

 mentioned. How rapidly the parents must reproduce in 

 order to bring forth this number will depend on the length 

 of life of the parents. 



In general, we may say that the rate of reproduction 

 in any species that has stood the test of time and has 

 proved successful does meet these conditions. Those that 

 care for their young need not produce so many; those 

 that have few enemies need not produce so many. But a 

 pair must on the average produce rapidly enough, live 

 long enough, and care for their offspring enough to bring 

 two to maturity during their own life. Otherwise the 

 species is doomed. 



3. Some Examples of the Rate of Reproduction. While 

 all organisms must thus produce more offspring than can 

 hope to survive, they differ greatly in the actual number 

 and in the rate of reproduction, because the other factors 

 enter in so differently. For example, an organism that 

 produces offspring which are large at birth, as the 

 mammals, do not meet so many failures in bringing 

 offspring to maturity as does a fern plant in which the 

 reproductive body is a small and delicate spore. In 



general, there are two ways to increase the output of 

 offspring: either by more frequent reproduction, or by 

 increasing the number produced at a time, or by a 

 combination of these. 



a. Bacteria. Some species of bacteria may divide in 

 half an hour. There are here only two offspring 

 resulting, but the divisions may come so frequently 

 that the bacteria multiply right up to the limit of 

 their food in a short while. Because of poisons 

 which they produce, and because the food at atiy 

 one place soon gives out, they are unable to keep 

 up this rate long at a time. This rapidity of 

 reproduction is very valuable to them, since the 

 decaying food in which they thrive does not last 

 very long. This power enables them to make the 

 most of it. At this rate of increase, multiplying 

 by two every half hour, in two days it would 

 require twenty-eight figures to represent the 

 individual bacteria resulting from one parent; the 

 total would be two raised to the forty-eighth power. 



b. Paramecium. Many of the one-celled animals live 

 on the same kind of decaying material as bacteria. 

 Some of them multiply at a rate nearly equal to 

 that in bacteria. Most, however, are not so rapid. 

 Paramecium, under favorable conditions, requires 

 from twelve to thirty hours to divide and grow to 

 maturity, but if its food held out and it could keep 

 up this rate, it would require only a few weeks for 

 the descendants of one Paramecium to make a 

 mass of protoplasm as big as the earth itself. 



c. Ferns. The fern produces spores only once a year, 

 but it produces millions at each season. If every 

 spore out of a million should succeed in producing 

 one mature fern plant, on the tenth year we would 

 have 1,000,000 to the tenth power of new fern 

 plants, to say nothing of all the old ones that lived 

 over from previous years. A single fern plant 

 could populate the earth with ferns alone in a few 

 years. 



d. The House Fly. Chancellor Jordan, of Leland 

 Stanford University, says concerning the reproduc- 

 tive power of this pest: "If all the eggs of a common 

 house fly should develop, and each of its progeny 

 should find the food and temperature it needed, 

 with no loss and no destruction, the people of a 

 city in which this might happen could not get away 

 soon enough to escape suffocation by a plague of 

 ffies." 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 sta't 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 has done. 



