OF FIRST CROSSES AND OF HYBRIDS. 273 



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

 ference 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 gov- 

 erning the facility with which trees can be grafted on each 

 other as incidental on unknown differences in their vegeta- 

 tive systems, so I believe that the still more complex laws 

 governing the facility of first crosses are incidental on un- 

 known differences in their reproductive systems. These 

 differences in both cases follow, to a certain extent, as 

 might have been expected, systematic affinity, by which 

 term every kind of resemblance 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 spe- 

 cial endowment; although in the case of crossing, the diffi- 

 culty is as important for the endurance and stability of 

 specific forms as in the case of grafting it is unimportant 

 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 blending, on the same principle 

 that, when man is selecting at the same time two varieties, 

 it is necessary that he should keep them separate. In the 

 first place, it may be remarked that species inhabiting dis- 

 tinct 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 compatriot, sterility with other species 

 would follow as a necessary contingency. In the second 

 place^ it is almost a,§ much opposed tq the theory of natural 



