ANIMALS AND THEIR ENVIRON .\rENT 289 



factors that influence the hfe of the animals. If the net result of all 

 these elements favors the species, it is successful. 



An important part of the success of a species is the numl)er of indi- 

 viduals it is prepared to pit against any unfavorable features of the 

 environment. This number depends first of all on the rate of repro- 

 duction. In this activity, animals differ greatly. The larger mammals 

 produce as a rule only one at a birth, and the period of development is 

 long, so that successive offspring are separated by wide intervals of time. 

 Rate of reproduction is slow in such animals. Contrast with them the 

 small mammals. A mouse produces half a dozen at a litter, and several 

 litters in a year, at which rate only a few years would he rec^uired for 

 the descendants of one pair to overrun the earth. A shad may lay 

 100,000 eggs in a year, a tapeworm 100,000 eggs per day. A protozoon 

 could, in seven years, produce a mass of protoplasm ten thousand times 

 as large as the earth. One aphid could in a single summer gi\'e rise to 

 500 thousand million million descendants. Punnett has calculated that 

 a female rotifer^-which is parthenogenetic, lays 50 eggs, and requires 

 only two days to reach maturitj^ — would be able to produce in a single 

 year, if all its potential offspring survived, a mass of rotifers large enough 

 to (ill the whole known universe and leave some over. 



In bisexual animals, the sex ratio is significant in the maintenance 

 of numbers, since the number of offspring is determined primarily by 

 the females. A species with many females has an advantage over one 

 with few. A short life history also favors large numbers, because there 

 w ill be more generations in a given time. 



Every species having great powers of reproduction is subject to 

 enormous destruction. This is proved by the fact that it does not, in 

 the long run, increase in numbers. Indeed, it may actually decrease. 

 The rotifer for which the foregoing calculation was made, once an abun- 

 dant object of biological experimentation, seems now to elude collection 

 altogether; and the passenger pigeon, exceedingly abundant over most 

 of eastern North America only a few decades ago, is now extinct. What 

 keeps a species in check is not easily ascertained. Accidents reduce 

 numbers to some extent, while predatory animals, disease, parasites, lack 

 of food, and unfavorable climatic or other physical conditions must 

 account for other extensive losses. The efficienc}^ of a species in over- 

 coming these obstacles determines its success. Rapid increase is not 

 always a sign of efficiency, for species which become especially abundant 

 in one season or over a period of several years must usually suffer a 

 reverse later; and there are circumstances (such as exhaustion of their 

 food) in which the greater the increase the more severe the following 

 decline. Greater safety lies in a steady maintenance of numbers. This 

 principle is illustrated by one of the most successful of birds, the English 



