NATURAL SELECTION; SEXUAL SELECTION 



61 



the necessity of the robin to find water in summer or to keep 

 warm in winter. All three forms of the struggle for existence, 

 intraspecific, interspecific, and environmental, are constantly 

 operative and with every species. In some regions or under 

 some conditions the one phase may be more destructive, in 

 others another. Any one of these may ])e in various ways 

 modified or ameliorated. When the conditions of life are most 

 easy, as with most species in the tropics, tliere the conflict of 

 individuals and the conflict of species is most severe. It is not 

 possible to say that any one of these three forms of struggle and 

 selection is more potent than the others. In fact, the first and 

 the second are in a sense forms of the third. All struggle is, 

 strictly speaking, with the conditions of life. Those individuals 



Fig. 38. — Praying mantis, eating a grasshopper. (Adapted from photograph 



from Ufe by Slingerland.) 



which endure this struggle survive to reproduce themselves. 

 The rest die and leave no progeny. 



Because of the destruction resulting from the struggle for 

 existence, more individuals in each species are born than can 

 mature. The majority fail to reach maturity because for one 

 reason or another they cannot do so. All live that can. Each 

 animal tries to feed itself: many try to take care of their young. 

 But in self protection and in propagation of the species very few 

 individuals succeed in comparison with the vast number which 

 the process of reproduction calls into being. 



The destruction in nature is not indiscriminate. In the 

 long run and for the most part, those creatures least fitted to 

 resist are the first to perish. It is the slowest animal wliich is 

 soonest overtaken by the pursuers. It is the weakest which is 



