112 Plant-Breeding 



the same two species. "Mirabilis jalapa can easily be 

 fertilized by the pollen of M . longiflora, and the hybrids 

 thus produced are sufficiently fertile ; but Kolreuter tried 

 more than two hundred times during eight following years 

 to fertilize reciprocally M. longiflora with the pollen of 

 M. jalapa and utterly failed," as have also many other 

 hybridizers. Frequently very closely related species 

 absolutely refuse to cross. This is true of the pumpkin 

 (Cucurbita Pepo) and squash (C. maxima). It is, never- 

 theless, true that hosts of very distinct species hybridize 

 readily, and a number of cases are known of species be- 

 longing to different and quite distinct genera having 

 hybridized, producing the so-called bigeneric hybrids. 

 For example, wheat and rye, and wheat and barley, be- 

 longing to closely related genera, cross with difficulty, and 

 Luther Burbank is said to have succeeded in obtaining a 

 hybrid of strawberry and raspberry. Bigeneric hybrids 

 are many among the orchids, even though they are highly 

 specialized plants ; and some trigeneric hybrids are known. 



Hybrids between plants belonging to different families 

 are very rare. The results obtained by hosts of experi- 

 menters and practical gardeners show conclusively that 

 the greater part of closely related species can be readily 

 crossed, while very distinct species, and species belonging 

 to distinct genera, can be crossed in only comparatively 

 few cases. It is impossible to predict what plants may or 

 may not be hybridized. 



Vigor as a result of crossing. Darwin was the first to 

 show that crossing within the limits of the species or 

 variety results in a constant revitalizing of the offspring, 

 and that this is the particular ultimate function of crossing 



