CH. XV] SUMMARY OF CONCLUSIONS 193 



28. With its probably genetical basis, it explains the difficulty 

 of the perfect form in which characters, and especially those of 

 the higher divisions, are exhibited, which was almost impossible 

 to the theory of natural selection. 



29. It gets over the difficulty caused by the fact that few tran- 

 sition stages are found, either in living or in fossil plants. 



30. It explains the universal hollow curve, as well as age and 

 area and size and space, all impossible to the theory of natural 

 selection. 



And one may add : 



The 34 test cases given often bring out new and sometimes 

 unexpected relations, e.g. the grouping of a family (or sub-families 

 if large) into large, medium, and small genera. 



Adaptation, isolation, and other phenomena are discussed from 

 somewhat new points of view. 



Upon pp. 76, 139, and elsewhere, indications have been given, 

 more or less unintentionally, about things that will only appear 

 in a forthcoming book upon geographical distribution. Therein 

 the writer hopes to show that the adoption of age and area and 

 of differential or divergent mutation, for both of which good 

 proof has now been given, reduces the problems of distribution 

 to a simpler form. By abandoning the supposition, necessarily 

 inherent in natural selection, that plants may be divided into 

 successes and failures, the one expanding and the other contract- 

 ing the area occupied, all may be regarded as behaving in much 

 the same way as their near relatives. One thus obtains a more 

 satisfactory picture of how evolution and geographical distribu- 

 tion went on, and how thev fitted into one another. 



WED 13 



