The Evidence from Hybrids. 119 



of the same species, and the study of crosses and hybrids 

 is therefore a means of separating, to some extent, the 

 influence of one parent from the influence of the other. 

 This is true, however, only with reference to character- 

 istics which are of recent acquisition, for the greater 

 part of the history of two allied species has been the 

 same, and they show in common everything except what 

 has been acquired by each one since they diverged from 

 their common ancestor. 



Crossing gives no way of showing whether these com- 

 mon characteristics are or are not transmitted by one 

 parent or the other or by both, but it does give us this 

 information regarding characteristics w r hich appear in 

 one species but not in the other, and it is therefore the 

 best means at our disposal for studying the influence of 

 each parent upon the offspring. 



Crossing as a Cause of Variation. 



According to our theory of heredity, we can easily see 

 how the crossing of two species or varieties should lead to 

 variability, for when two species or varieties are crossed 

 certain cells of the body will be hybrids between the 

 gemmules of the male parent and the ovarian particles 

 inherited through the female from the egg of the pre- 

 ceding generation. Now the ovarian particle transmits 

 the properties of a cell like that of the female parent, 

 while the gemmule transmits those of a corresponding 

 cell in the father. It is plain that corresponding cells 

 of a female of one species or variety and of a male of 

 another species or variety must be more different from 

 each other than corresponding cells in a male and female 

 of the same species or variety. The hybrid cell formed 

 by their union would, therefore, be expected to differ 

 more from each of them, that is, to vary more than it 



