HYBRIDS 135 



parent species rarely produce offspring, though they some- 

 times do so. The best cases of the production of fertile 

 hybrids are between species of flowering plants brought 

 to this country from widely separated regions. The 

 surprising and instructive result has been obtained that 

 a cross between two allied species (that is, of one and 

 the same " genus ") which will fail altogether or " come 

 to nothing " as infertile hybrids — if the two species crossed 

 are from the same or contiguous regions — yet will yield 

 readily vigorous fertile hybrid offspring when the two 

 species (always, of course, of one and the same genus) 

 have their native homes in widely separate parts of the 

 world — as, for instance, the Indian Himalaya range and 

 the South American Andean range. 



This has been found in crossing species of rhodo- 

 dendrons, of orchids, and of many other plants with which 

 horticulturists occupy themselves for commercial purposes. 

 It is in some ways the reverse of what one might expect. 

 It would be reasonable to suppose that allied species from 

 the same climate and geographical region would have 

 more affinity and be more readily hybridized than species 

 from widely remote and physically differing regions. But 

 the reverse is the case, many thriving hybrid stocks which 

 duly fertilize and set their seed are now in cultivation, 

 having been produced by the union of parent species 

 from " the opposite ends of the earth." 



The consideration of this case throws some light on 

 the significance of the non-occurrence of natural hybrids 

 and of the very remarkable and curious fact that hybrids 

 are so usually sterile. When we come to think of it, the 

 natural preliminary assumption should be (as is that of 

 unsophisticated humanity) that any animal or plant 

 might, so far as possibilities go, breed with any other ; 

 and the questions to be answered are: (1) What advantage 

 to a species is it not to be able to hybridize with other 



