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which are not closely allied are characterised by a consider- 

 able degree of sterility and are often entirely sterile. As 

 fertilisation consists essentially in the union of two sexual 

 cells, a portion of the protoplasm of each cell entering into 

 the constitution of the embryo which results from such a 

 union, we should naturally expect the distinguishing charac- 

 teristics of each parent to become more or less apparent 

 in the hybrid. This is the case and the hybrid is found either 

 to be intermediate between the parents, the characters of 

 both parents being combined in it, or else to resemble one 

 parent more than the other. Occasionally also it may pre- 

 sent entirely new characters. In some cases the result of 

 hybridisation is different according to which parent is chosen 

 as the father or mother respectively. Thus a cross may 

 only be possible when an individual of a certain species is 

 always chosen as the father, the reciprocal cross with the 

 same individual as the mother being sterile. 



Again, the hybrid in some cases is always found to re- 

 semble one parent, e.g. the father, more than the other. Fertile 

 hybrids are those which, when fertilised by their own 

 pollen or by that of another individual of the same hybrid 

 cross, produce fertile seed from which fertile plants are 

 developed. Fertile hybrids may be constant, as regards their 

 distinguishing characters, from the first, but in the majority 

 of cases they are found to split, i.e. to revert to the parent 

 forms in subsequent generations. In such cases of reversion, 

 however, a few individuals are often found which remain 

 constant hybrids, with characters distinguishing them from 

 both parents. A hybrid may be successfully crossed not 

 only with another hybrid but with one of its parents ; if this 

 is repeated, the derivative hybrid obtained from such a cross 

 being again crossed with the same parent and so on, the off- 

 spring are found to resemble that parent more and more, 

 and finally to completely revert to it. The hybrid, instead 

 of being crossed with one of its parents, may be crossed with 

 an individual belonging to an entirely distinct species and 

 this operation may also be repeated. In this way it has 

 been found possible to obtain a derivative hybrid in which 

 the characters of six, or even more, different species are com- 

 bined. Hybrids occur in nature but not very frequently ; 

 probably this is partly due to the fact that, in the majority 

 of cases, where the flowers of a plant are simultaneously pol- 

 linated by different plants, the pollen o an individual belong- 

 ing to the same species is prepotent, i.e. it is able to accom- 



