128 REPRODUCTION 



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 any 

 one place soon gives out, they are unable to keep 



