390 BIOLOGY AND ITS MAKERS 



number of any one kind remains practically constant. 

 Although the face of nature seems undisturbed, there is 

 nevertheless a struggle for existence among all animals. 



This is easily illustrated when we take into account the 

 breeding of fishes. The trout, for illustration, lays from 60,000 

 to TOO,ooo eggs. If the majority of these arrived at maturity 

 and gave rise to progeny, the next generation would represent 

 a prodigious number, and the numbers in the succeeding 

 generations would increase so rapidly that soon there would 

 not be room in the fresh waters of the earth to contain their 

 descendants. What becomes of the immense number of 

 fishes that die ? They fall a prey to others, or they are not 

 able to get food in competition with other more hardy rela- 

 tives, so that it is not a matter of chance that determines 

 which ones shall survive; those which are the strongest, the 

 better fitted to their surroundings, are the ones which will 

 be perpetuated. 



The recognition of this struggle for existence in nature, 

 and the consequent survival of the fittest, shows us more 

 clearly what is meant by natural selection. Instead of man 

 making the selection of those particular forms that are to 

 survive, it is accomplished in the course of nature. This is 

 natural selection. 



Various Aspects of Natural Selection. — Further illustra- 

 tions are needed to give some idea of the various phases of 

 natural selection. Speed in such animals as antelopes may 

 be the particular thing which leads to their protection. It 

 stands to reason that those with the greatest speed would 

 escape more readily from their enemies, and would be the 

 particular ones to survive, while the weaker and slower ones 

 would fall victims to their prey. In all kinds of strain due to 

 scarcity of food, inclemency of weather, and other untoward 

 circumstances, the forms which are the strongest, physio- 

 logically speaking, will have the best chance to \Veathcr the 



