EVOLUTION. 415 



are details which are still in dispute. Some of the factors 

 may be outlined here. 



Fertility and the Struggle for Existence. Animals tend to 

 multiply in a geometrical ratio, the number of individuals 

 in any generation being the number in the preceding 

 multiplied by the number of young produced. This 

 rapidly results in enormous numbers. Thus Darwin has 

 estimated that the progeny of a single pair of elephants, the 

 slowest breeders among animals, would number 19,000,000 

 in about 750 years. In other animals the rate is far more 

 rapid. The dogfish (Acanthias) breeds at the age of two 

 years and produces, on the average, six young at a birth. 

 The English sparrow breeds four times a year, laying six 

 eggs in a clutch. The full-grown codfish produces over 

 a million eggs a year. With many lower animals the rate 

 of increase is even greater. Maupas states that if the 

 Protozoan he studied were to continue to reproduce at 

 its most rapid rate, the result, in thirty-eight days, would 

 be a mass of Protozoa equalling the sun in size. 



We know, however, that there is no such actual increase 

 in the number of individuals. In fact, from year to 

 year the number of any species varies but little. In 

 other words, the great majority of eggs or young pro- 

 duced fail to come to maturity, but die at an early age. 

 Only a small minority survives. There is constantly a 

 struggle for existence with every species, and it is evident 

 that, in the long run, only those best fitted for their sur- 

 roundings will survive, while those less fit will perish. 

 In fact, even a very slight difference may determine the 

 fate of the individual. This struggle may be between 

 (1) individuals of the same species; (2) between different 

 species which prey upon or serve as food for each other; 



