CHAPTER VI. 



THE EVIDENCE FROM HYBRIDS. 



Importance of the subject It furnishes a means of analyzing or 

 isolating the influence of each sexual element Hybrids very 

 variable Hybrids from domesticated races more variable than 

 those from wild races The descendants of hybrids more varia- 

 ble than the hybrids themselves The offspring of a male hybrid 

 and the female of a pure species are much more variable than 

 those of a female hybrid and the male of a pure species These 

 facts inexplicable on any view, except the one here presented 

 Reciprocal crosses They differ in fertility and in structure 

 The difference is exactly what our theory requires Diffi- 

 culty in explaining transmission of characters without fusion 

 Reversion caused by crossing Two kinds of reversion Sum- 

 mary. 



THE study of hybrids and crosses is of especial interest 

 to us, since it affords us a means, somewhat imperfect 

 it is true, for recognizing, in the offspring, the structure 

 which it owes to each parent. 



In ordinary sexual reproduction between animals or 

 plants of the same race, the parents are almost exactly 

 alike, except for their sexual differences; and as nearly 

 every structural feature of the young is a feature of re- 

 semblance to each parent, there can be nothing to show 

 that it is inherited from the one rather than from the 

 other. 



When distinct races or species arc crossed, the case is 

 somewhat different. It is true that the two parents are 

 still very much alike, for species cannot be made to breed 

 together at all unless they are very closely related. Still 

 they are more different from each other than individuals 



