252 PRINCIPLES OF ANIMAL BIOLOGY 



in the monastery at Briinn, in Austria, in the latter half of the last cen- 

 tury, found time to experiment with garden peas on the monastery 

 grounds. He knew a number of varieties which differed from one another 

 in the height of their stems, the form and color of the seeds, the shape and 

 color of the pods, and the position of the flowers. From crosses between 

 these varieties Mendel derived the simple law of heredity which bears 

 his name. He published the results of his experiments in 1865 and 1866, 

 but they attracted no attention. Probably because biologists were 

 engrossed with Darwin's theory of Natural Selection, which had been 

 announced a few years before, Mendel's work lay unnoticed for a third of 

 a century. Then, in 1900, three European biologists, working indepen- 

 dently, rediscovered the principle which Mendel had published. At the 

 same time Mendel's paper was brought to light, and his three successors 

 in this field generously gave him the credit that was his due. Since 1900 

 Mendel's Law has been abundantly verified. Although his scheme of 

 inheritance has been elaborated since that time, the fundamental feature 

 of it is now generally recognized as applicable to the bulk of the known 

 facts regarding heredity. 



Mendel's Law. — The fundamental feature of Mendelian heredity is 

 the segregation of the genes in the production of the germ cells. The two 

 genes concerned with a given character separate from one another, going 

 to different germ cells. Different pairs of genes sggrega^e, at least 

 frequently, independently of each other, so that genes may be brought 

 together in the germ cells in combinations that never existe,d before. 

 The fertilization of eggs by sperms may introduce further variety into 

 the combinations, for in the fertilized egg the variety among the ova is 

 multiplied by that of the sperms. As a result, the inherited characters 

 of animals may be grouped in ever new combinations. Because such 

 inherited traits as color of hair, color of eye, length of ear, length of hair, 

 and direction of hair can be combined in new ways in hybrid individuals, 

 they have sometimes been called unit characters. That these characters 

 behave as units is due to the segregation, resulting in new combinations, 

 of genes in the germ cells. This is the essential feature of Mendel's 

 Law. That law does not require dominance, for there are cases in which 

 dominance is lacking but all other phenomena are typical. No particular 

 ratios, such as 3 : 1, are necessary, since many things may conspire to alter 

 these ratios. Mendel's Law refers to the segregation and recombination 

 of the genes. 



Determination of Sex. — Closely related to heredity are the phenomena 

 of sex-determination. What causes some embryos to develop the organs 

 characteristic of a male, while other embryos that look precisely like 

 them in early stages develop female organs? The answer to this question 

 is not the same for all animals, and for many of them the question is still 

 unanswered. One explanation, however, is applicable to a large number 



