THE EARTH FOR MANKIND 537 



picks out what she wants to preserve, or favors one group at 

 the expense of another, but to emphasize a process which is 

 evident enough, in order to offer a possible explanation for 

 something that is not so clear. For example, we know that in 

 different parts of the world there are species of animals or plants 

 that are of the same family but different in many details. The 

 grasses of the tropics are different from those of the temperate 

 zones, and these differ from those farther north or south ; and 

 each species seems to fit better into its surroundings than some 

 related but different species would. The same is true of ani- 

 mals. Another important fact is that the species of plants and 

 animals that lived in past ages resemble those of today in some 

 respects, yet are different. We have seen that in the course of 

 years the cultivators of plants and animals have gradually 

 brought about changes in the domestic varieties of organisms. 

 It was artificial selection that gave us tumbler pigeons and fine 

 poultry, high-grade wheats and many varieties of cabbage. 

 Now Darwin supposed that a similar process takes place in na- 

 ture ; the less favored individuals are destroyed and the more 

 favored ones (that is, those better adapted to their life condi- 

 tions) survive and leave offspring. There is always a great deal 

 of individual variation, and more individuals are always started 

 than can possibly reach maturity. This selection, generation by 

 generation, ought in the course of long ages to establish in each 

 locality those varieties that were best adapted to the conditions. 



We have seen (sect. 367) that many of the variations among indi- 

 viduals are not inherited. The selection would have to take place in each 

 generation if the qualities were to be preserved or accumulated. There 

 are other qualities, however, which are inherited and which may help in 

 survival. Moreover, there are probably mutations of some degree ap- 

 pearing all the time, and these may play an important part in the grad- 

 ual change of species as time goes on. In Darwin's time it was not 

 realized either that the ordinary fluctuations were not inherited or that 

 mutations are so frequent and more stable. 



388. Man's struggle. As a living being man has to meet the 

 conditions of life just as truly as do other organisms. In some 



