HYBRIDISM 251 



when on their own roots. We are reminded by this latter fact of 

 the extraordinary cases of hippeastrum, passiflora, etc., which 

 seed much more freely when fertilized with the pollen of a distinct 

 species than when fertilized with pollen from the same plant. 



We thus see, that, although there is a clear and great difference 

 between the mere adhesion of grafted stocks and the union of the 

 male and female elements in the act of reproduction, yet that 

 there is a rude degree of parallelism in the results of grafting 

 and of crossing distinct species. And as we must look at the 

 curious and complex laws governing the facility with which trees 

 can be grafted on each other as incidental on unknown differences 

 in their vegetative systems, so I believe that the still more com- 

 plex laws governing the facility of first crosses are incidental on 

 unknown differences in their reproductive systems. These differ- 

 ences in both cases follow, to a certain extent, as might have been 

 expected, systematic affinity, by which term every kind of re- 

 semblance and dissimilarity between organic beings is attempted 

 to be expressed. The facts by no means seem to indicate that the 

 greater or lesser difficulty of either grafting or crossing various 

 species has been a special endowment; although in the case of 

 crossing, the difficulty is as important for the endurance and 

 stabihty of specific forms as in the case of grafting it is unim- 

 portant for their welfare. 



ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES 

 AND OF HYBRIDS 



At one time it appeared to me probable, as it has to others, 

 that the sterility of first crosses and of hybrids might have been 

 slowly acquired through the natural selection of slightly lessened 

 degrees of fertility, which, like any other variation, spontaneously 

 appeared in certain individuals of one variety when crossed with 

 those of another variety. For it would clearly be advantageous to 

 two varieties or incipient species if they could be kept from blend- 

 ing, on the same principle that, when man is selecting at the same 

 time two varieties, it is necessary that he should keep them sepa- 

 rate. In the first place, it may be remarked that species inhabiting 

 distinct regions are often sterile when crossed ; now it could clearly 

 have been of no advantage to such separated species to have been 

 rendered mutually sterile, and consequently this could not have 

 been effected through natural selection; but it may perhaps be 

 argued, that, if a species was rendered sterile with some one com- 

 patriot, sterility with other species would follow as a necessary 

 contingency. In the second place, it is almost as much opposed 



