CHAPTER VI. 



THE EVIDENCE FROM HYBRIDS. 



Importance of the subject — It furnishes a means of aualj'zing 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 h3'brids more varia- 

 ble than the liybrids 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 

 — Tlie 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 sttidy of hybrids and crosses is of especial interest 

 to ns, since it affords ns a means, somewhat imperfecl. 

 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 neaily 

 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 are crossed, the case is 

 somewhat different. It is true that the tAVO parents arc 

 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 



