82 SEXUAL SELECTION. 



with its own kind. The inevitable result is the survival of 

 the fittest^ i.e.^ those which are best adapted to their environ- 

 ment live and reproduce their kind, and the less fit die early. 

 The net result is a selection of the superabundant offspring 

 which, as it occurs throughout nature, has been termed 

 Natural Selection, Darwin's theory of evolution by Natural 

 Selection therefore depends upon two main principles or 

 natural phenomena : 



1. Variation, or the structural differences between a parent and 



its offspring. 



2. The struggle for existence, due to production of offspring above 



the means of subsistence. 



The second principle acting upon the first must result in a 

 selection of the variations. All variations which tend to 

 higher efficiency are preserved and intensified through many 

 generations till a fresh species is produced. 



Animals under domestication are not, as a rule, subjected 

 to a struggle for existence, and hence there is no natural 

 selection. Man has, however, persistently selected the 

 variations which appealed to his fancy, and by this artificial 

 selection has been enabled to produce the numerous breeds 

 of dogs, horses, cattle, pigeons, rabbits, &c. In this case 

 the mental faculties of man perform the selective opera- 

 tion which is automatically effected in nature by the fierce 

 struggle for life. 



It is questionable if these artificially produced "breeds" are really 

 comparable to the natural "species" for, if the breeds are left to them- 

 selves, rapid intercrossing results, in a few generations, in the 

 disappearance of the "breed" characteristics and a reversion to the 

 primitive ancestors from which they were originally derived. 



Sexual Selection. — In certain cases, especially among 

 the higher animals, the female individuals exercise a selective 

 faculty among the males. Contests of various kinds take 

 place among the males, and the successful competitors 

 alone pair with the females. This form of natural selection, 

 termed sexual selection, probably accounts for the production 

 of the secondary sexual characters referred to in Chapter V. 

 (See page 44.) 



Let us apply the Darwinian theory of evolution to 

 the case of oceanic islands referred to in Chapter VI I. (p. 64). 

 Suppose a number of winged insects have been blown by a 



