BEES AND FRUIT-GROWING 



But for plants which are actively developing and are forced 

 into fierce competition or are compelled to meet new conditions 

 crossing is indispensable. Darwin sowed crossed and self- 

 fertilized seeds on the opposite sides of small pots so that there 

 was a struggle for bare existence; and the crossed plants grew 

 more vigorously, bloomed earlier, and more profusely, and pro- 

 duced more seed-capsules. Thus the inbred races tend to dis- 

 appear. 



Crossing is by no means confined to the individuals and 

 varieties of the same species, but is very common between dis- 

 tinct species and may occur between difi^erent genera. It is 

 rapidly coming to be regarded as an important factor in evolu- 

 tion. In Kerner's time more than 1,000 hybrids were known in 

 the flora of Europe, and he fully believed that many new species 

 originated in this way. Darwin had previously realized the 

 possibility that hybridism might have played an important part 

 in the history of evolution; but owing to the general belief that 

 hybrids were almost invariably sterile he underestimated its 

 significance, although he observed that every intermediate 

 stage existed between complete sterihty and complete fertility. 



^Vhile hybrids do in general show decreased fertility, there 

 are thousands of cases in which they multiply readily by seed. 

 Jeffrey has recently shown that hybrids among the Angiosperms, 

 or flowering plants, are characterized by having a part of the 

 pollen imperfect or aborted; and, judged by this test, they are 

 very common both among wind-pollinated and insect-pollinated 

 plants. A great many forms which have long been regarded 

 by systematists as perfectly good species are now recognized 

 by their aborted pollen as hidden hybrids. They are especially 

 abundant in the rose family among the roses, apples, pears, 

 brambles, and hawthorns. 



Among wind-pollinated flowers hybrids are very common in 



