HEREDITY AND EVOLUTION 181 



The last two types represent combinations of characters 

 which did not exist in either the parents or the grandparents 

 and which are different from conditions in any of their an- 

 cestors. In the offspring two characters which were origi- 

 nally associated in the ancestors may become separated and 

 act entirely independently of each other. In breeding to- 

 gether members of the Fi generation there is just the same 

 chance for black to become associated with long hair as for 

 it to remain with its original associate, short. 



Many new varieties of plants and animals have been 

 produced by new recombinations of characters like the ex- 

 ample just cited. It was chiefly by methods of this sort 

 that Burbank produced his many varieties of fruits and 

 vegetables. 



In the foregoing pages we have seen how new combina- 

 tions of characters are readily explainable on the basis of 

 definite laws of heredity. Further, we have observed the 

 readiness with which man at his own pleasure creates new 

 types of plants and animals by selection and by crossing 

 individuals showing either slight or pronounced desirable 

 variations. The question now arises as to the relation which 

 exists between the differences among animals produced 

 under man's direction and observation and the differences 

 among animals in nature. 



Fossil Animals. Students of fossil life of past ages (paleon- 

 tologists) tell us that there are but few species of animals 

 living today which are the same as those existing in the 

 distant past of the earth's history. The early layers of the 

 crust of the earth contain no remains of what we today 

 speak of as the higher animals, or vertebrates (fishes, rep- 

 tiles, birds, and mammals). Furthermore, these various 

 forms of vertebrates did not all appear at once. There was 

 a time when fishes were the highest type of animal living on 

 the earth. In later fossil deposits remains of amphibians, 



