278 BIOLOGY: GENERAL AND MEDICAL 



4. The atrophy and gradual disappearance of useless 

 organs and appendages in species that survive in 

 the struggle for existence. 



5. Heredity. The characters of the species and 

 special characters of the parents reappear in the 

 offspring, thus tending to preserve those varia- 

 tions that prove useful in the struggle for 

 existence. 



6. Sexual selection. The contest among males for 

 the possession of the females, in which the poorer 

 and weaker will be eliminated. The relative 

 attractiveness of one sex for the other results in 

 the preservation or elimination in sexual dimor- 

 phism. 



The chief factor he considers to be "natural selection'' 

 or t( struggle for existence," which he finds to be identical 

 with Herbert Spencer's doctrine of the "Survival of the 

 fittest." 



Darwin begins by a consideration of the various 

 "breeds" of domestic animals and shows that from 

 a few primitive stocks the many varieties of domestic 

 animals have been cultivated by artificial selection. He 

 points out that among animals there are slight variations 

 in the direction of desirability and undesirability, and 

 that by carefully conserving the desirable and eliminat- 

 ing the undesirable, man has been able to produce the 

 various kinds of cattle, sheep, hogs, horses, dogs, rabbits, 

 fowls, pigeons, etc., so well known to us. If it is to be 

 conceived that natural selection is analogous to artificial 

 selection, it is first necessary to admit that living things 

 of the same kind vary under natural conditions. Con- 

 cerning this, he says : 



"The many slight differences which appear in the offspring 

 from the same parents, or which it may be presumed have thus 

 arisen, from being observed in the same locality, may be called 

 individual differences. No one supposes that all the individuals 

 of the same species are cast in the same actual mould. These 

 individual differences are of the highest importance to us, for they 

 are often inherited, as must be familiar to everyone; and they 

 thus afford materials for natural selection to act on and accumulate 



