158 THE THEORY OF EVOLUTION 



fraction of a single lecture, in which only a hasty and 

 most inadequate sketch can be attempted. On the 

 other hand, it is not possible to omit all mention of 

 Mendel's work from a review of the modern state of 

 opinion on the subject of evolution, for this work has 

 thrown great light upon the mechanism of heredity 

 and, as has been repeatedly insisted, heredity is an 

 all-important part of evolution. 



Mendel crossed two varieties of peas, one of which 

 constantly had white flowers and the other violet 

 flowers. The first generation of hybrid plants all 

 had violet blossoms, which Mendel explained by 

 saying that in this particular cross the violet colour 

 was ;< dominant" over the white, which was '* re- 

 cessive." The second generation of the hybrids, 

 however, showed a change, only three-fourths of the 

 plants had violet blossoms and one-fourth had white; 

 the latter, in all subsequent generations, gave only 

 white flowers; the plants had "reverted" to the origi- 

 nal white parent. Of the violet-flowered plants in the 

 next and all following generations, one-third re- 

 mained constantly violet and two-thirds split in the 

 next generation into three-fourths violet and one- 

 fourth white. From the mere inspection of a violet- 

 flowered plant, it was impossible to say whether it 

 would give rise only to violet offspring, or to those 

 which were partly violet and partly white. These 

 and thousands of other experiments indicate that an 

 organism is a mosaic of 'unit characters," which are 

 transmitted independently of one another to the 



