No. 591] HYBRIDISM IN ANGIOSPEBMS 143 



climatic differentiation, and most important of all with 

 the phenomenon of heterozygosis. 



In conclusion the situation may be summarized. The 

 phenomenon of variation in the older types of plants is 

 still unexplained and must apparently be accepted as an 

 ultimate characteristic of living matter. In the case of 

 those groups of plants, which have achieved predomi- 

 nance under the present climatic conditions of our earth, 

 hybridism has clearly played a large role in the accelera- 

 tion of the processes of evolution. The peculiar condi- 

 tions presented by the species of Oenothera, which have 

 been put forward by De Vries in favor of his mutation 

 hypothesis, are obviously only a particular case of the 

 manifestation of the natural hybridism, which is so wide- 

 spread a feature of the Angiosperms. The mutation 

 hypothesis has suffered a process of rapid disintegration 

 of late and it is increasingly clear on the botanical side 

 that where the term mutation is used it ordinarily indi- 

 cates changes which are the result of previous hybridiza- 

 tion. Concerning the Animal Kingdom the trend of 

 opinion is apparently setting equally strongly against 

 mutation. My zoological colleague, Professor Castle, has 

 recently declared himself in no uncertain terms against 

 the hypothesis of mutation, an expression of opinion not 

 the less convincing because he originally held the view 

 that mutation was a necessary pendant to Mendelism. 

 He is now able to explain to himself the appearance of 

 new characters as a result of the summation of small vari- 

 ations, which is essentially the Darwinian position. 



