THE FLOWER AND THE BEE 



the sedges {Carex), rushes, pondweeds, oaks, and birches. 

 Among insect-polhnated flowers they abound among the orchids, 

 willows, violets, and roses; while more than 1,000 species of 

 brambles (Rubus) have been described in Europe, a large part 

 of which are probably hybrids. Many of the so-called species 

 of Crataegus, of which there seems to be no end, are the result 

 of crossing. They are likewise abundant among the mulleins, 

 gentians, nightshades, evening-primroses, thistles, hawkweeds, 

 and asters. 



There is not a year passes that cross-fertilization between 

 different species does not occur on a very extensive scale; 

 but owing to unfavorable climatic conditions or intense com- 

 petition few or none even of the fertile hybrids survive. Occa- 

 sionally a hybrid finds a suitable habitat and becomes a new 

 species, and in the course of the development of the flowering 

 plants their number has become very large. Hybrids are very 

 variable and the great variability of the Angiosperms is doubtless 

 due to the frequency with which crossing has taken place. Vari- 

 abihty in turn has hastened the development of new" species. 

 Hybridism has, in the opinion of Jeffrey, clearly played a large 

 role in the acceleration of the evolution of the flowering plants. 

 It is still an active agency the investigation of which offers more 

 promising results than any other factor in evolution. Its in- 

 fluence in the transformation of species may prove to be very 

 far-reaching. 



278 



