THE DOCTRINE OF EVOLUTION 105 



millions of stout men." A similar rapid increase would be 

 noted on the part of any animal, were the various checks to 

 its multiplication removed. The struggle for life goes on 

 between different individuals of the same species, as in a 

 swarai of locusts ; between individuals of different species, 

 as with locusts and insect-eating birds, and with the con- 

 ditions of existence, such as temperature, winds, moisture, 

 and food-supply. 



Owing to the struggle for existence, Darwin says: "Varia- 

 tions, however slight, and from whatever cause proceeding, 

 if they be in any degree profitable to the individuals of a 

 species, in their infinitely complex relations to other organic 

 beings and to their physical conditions of life, will tend to 

 the preservation of such individuals, and will generally be 

 inherited by the offspring. The offspring, also, will have a 

 better chance of surviving, for, of the many individuals of 

 any species which are periodically born, but a small number 

 can survive." 



The caddice-flies are, in their immature stage, aquatic 

 larvae which build protective cases composed of grains of 

 sand, or bits of straw, or leaves (Fig. 55). These cases afford 

 concealment and protection to the young. Applying the prin- 

 ciple of natural selection here, it would be said that those 

 caddice-flies which varied in the direction of protective cases 

 have survived, and those which did not have been devoured 

 or otherwise destroyed ; hence a race of case-building cad- 

 dice flies is in existence to-day. Natural selection results in 

 "the survival of the fittest" for the environment, and the 

 principle is used to explain the degeneration due to parasi- 

 tism, as well as the development of increased complexity in 

 animal structure. 



We have spoken of the conflict between individuals of the 

 same species for the necessities of existence, but many species, 

 individually rather weak, become successful competitors in 



