50 EVOLUTION OF TO-DAY. 



deny that these features are really reversions to a 

 common ancestor. 



Summary. 



If animals and plants produced young precisely 

 like themselves there would be no possibility for a 

 belief in evolution, for of course no new species could 

 arise by genetic descent. But a study of animals 

 under domestication, where they can be carefully 

 watched, as well as under nature, has proved that 

 animals do not produce young precisely like them- 

 selves. Two laws are found to govern animals in 

 their reproduction. The first is heredity; and in 

 accordance with this, species tend to repeat them 

 selves in their young. The second is variation ; and 

 in accordance with this law animals produce young 

 which are in many features unlike their parents. 

 These variations in the young are very numerous. 

 They are sometimes very small and sometimes very 

 great. They are transmitted from generation to 

 generation, and have a tendency to become greater 

 and greater. Each variation has of course its ade- 

 quate cause, either in the organism or in its external 

 conditions. Occasionally it is possible to discover 

 their causes, but in most cases it is impossible. 

 Plainly, if the same conditions continue to act upon 

 organisms for many generations, the variations they 

 produce will continue to increase. And this is found 

 to be true. By the accumulation of these variations 

 there are finally formed groups within the species. 

 Each group differs in certain respects from the 

 others, though they all show so much likeness that 

 they are still grouped together as a single species. 



