EVOLUTION OF ANIMALS AND PLANTS 571 



selection. The fact that natural selection acts only by sur- 

 vival of the fittest in the struggle for existence, limits its 

 effect to things which make animals and plants better 

 adapted to their environment. On the other hand, man can 

 control conditions so as to select for preservation under 

 domestication any peculiar individuals which interest him. 

 This is why we have such vast numbers of varieties of cul- 

 tivated plants and domesticated animals. 



501. Heredity. It is a well-known fact that offspring 

 tend to resemble their parents closely. We commonly speak 

 of characteristics which previously appeared in the parents 

 or even earlier progenitors as inherited or due to heredity. 



What may be Inherited. We have previously noted that 

 " like tends to produce like," which means that each animal 

 and plant has the power of transmitting its own general 

 characteristics to its offspring, e.g., a frog transmits frog 

 characteristics, a bird those of its own species, and so on. 

 Such characteristics which are part of the constitution of 

 organisms are often called germinal, which means, in the germ 

 (i.e., ova and sperm cells). 



Characteristics developed during the lifetime of any indi- 

 vidual, dating from the fertilized egg, are said to be acquired. 

 Any change in structure due to use or accident, e.g., develop- 

 ment of muscles by exercise, or loss of organs by accidents or 

 surgical operations, produces an acquired characteristic. 



Stating briefly the essential facts of heredity as now known, 

 characteristics acquired during the lifetime of individuals are 

 not transmitted in heredity, while germinal ones are capable 

 of inheritance. For example, the horns have been removed 

 from many cattle, the appendix from many human individuals, 

 tails from sheep and certain breeds of dogs ; and in no case 

 has the removal of any organ from a parent caused the birth 

 of offspring without the organ. In short, such acquired 

 characteristics are not inherited. On the other hand, a cow 

 which was germinally hornless (i.e., born without ability to 



