126 Heredity. 



functions of the two reproductive elements are alike, do 

 furnish convincing proof of the contrary, and show that 

 they are not alike. 



A reciprocal cross is a double cross between two spe- 

 cies or varieties, one form being used in one case as the 

 father, and in the other case as the mother. Thus a 

 reciprocal cross between a horse and an ass is a double 

 cross, between the male horse and the female ass on the 

 one hand, and the female horse and male ass on the 

 other. 



Now, if it is true that the function of the ovum is like 

 that of the male cell, the offspring of reciprocal crosses 

 should be alike in all respects, but this is by no means 

 the case. 



In the first place, the degree of sterility often differs 

 greatly in two species when reciprocally crossed; for the 

 male of the first will, iii some cases, readily fertilize the 

 ovum of the second, and thus give rise to descendants; 

 while hundreds of attempts to fertilize the ovum of the 

 first by the male of the second, result in uniform failure. 

 It often happens also that even when both crosses result 

 in the ]iroduction of offspring, the hybrid in the one case 

 is sterile, Avhile in the other case it is perfectly fertile. 



Not only do the results of reciprocal crossing show this 

 difference, but they show what is still less reconcilable 

 with the view that the functions of the sexual elements 

 are alike, namely, great differences of structure. 



In some cases where a reciprocal cross is i)erfectly fer- 

 tile on both sides, the hybrids which are thus produced 

 are not at all alike. When the male of species A and 

 the female of B are crossed, the offspring is an entirely 

 different being from the one born from A as a motlier 

 with B as a father. 



We know that allied species of animals are the descend- 



