HEREDITY AND EVOLUTION 177 



win." Mendel's work received little attention at first, but 

 a full generation later a Dutch scientist, de Vries, redis- 

 covered the work of Mendel. In 1900 Mendel's work be- 

 came generally known to the scientific world. Since that 

 date the study of heredity has been the most actively pur- 

 sued of all the branches of biology. There have been an 

 extremely great number of studies in which it has been 

 shown that heredity of given characters follows the prin- 

 ciples laid down by Mendel. In studying the operation of 

 heredity it has been discovered that some of the conditions 

 which we ordinarily think of as simple are really due to 

 several different causes. This has led to the use of the term 

 unit character to distinguish those traits or characters which 

 pass as indivisible units from one generation to its offspring. 

 According to Mendel's principles, when mating occurs be- 

 tween two animals of the same species (Fig. 98) differing in 

 but one of the many unit characters, the offspring will often 

 show not a condition intermediate between the two parents 

 but the character which appeared in only one of the parents. 

 Thus the offspring of one white and one black guinea pig 

 will be not gray but black, in color agreeing with one of the 

 parents, not intermediate between them. A character from 

 one parent which impresses itself in the offspring to the ex- 

 clusion of the contrasting character from the other parent is 

 said to be dominant. In contrast, the character which is not 

 seen in the offspring of the first generation (or F t ) is said to 

 be recessive. If experiments were not carried further than 

 one generation it might seem that the recessive character 

 is entirely lost. But when the experiment is carried further, 

 new facts are brought out. When the black guinea pigs, 

 whose parents were one black and the other white, reach 

 maturity and are bred together their offspring will be some 

 of them black and some of them white. The proportions 

 of the black and the white individuals in this second gen- 



