X76 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



facit saltum has been accepted as a grand canon by most naturalists, 

 and the evident absence of connecting-links has been thought fatal to 

 theories of evolution. My studies in plant life lead me to the belief 

 that one form will spring from another essentially different, and without 

 any gradual or insensible modifications uniting them." 



His conclusions are remarkably similar to those of Professor de Vries, 

 mentioned in the present and preceding volumes. They are the 

 following ' ' truths ' ' as Mr. Meehan called them : — 



1. Morphological changes in individual plants are by no means 

 always by gradual modifications. 



2. Variations from specific forms follow the same law. 



3. Variations are often sudden, and also of such decided characters 

 as to be deemed generic. 



4. These sudden formations perpetuate themselves, and act in all 

 respects the same as forms which spring through gradual modifications. 



5. Variations of similar character occur in widely separated localities. 



6. Variations occur in communities of plants simultaneously by 

 causes affecting nutrition, and perhaps by other causes. 



Arguing from these, new and widely distinct species may be 

 suddenly evolved from pre-existing forms without the intervention of 

 connecting-links. 



Hence two facts are required for mutations — some markedly strong 

 varietal characters, and constancy by heredity. 



How and when do mutations arise ? Professor de Vries is under the 

 impression that they do so periodically; that plants have periods of 

 mutation and periods of constancy. Then, variations come suddenly, 

 in every direction, while constancy may fix them as elementary species 

 if they do not differ much from their parents. They arise, according 

 to his view, in consequence of some latent property in the plant 

 suddenly coming into action. I have already called attention to his 

 apparent unfamiliarity with the conclusions of ecological experience, 

 and it is equally apparent in this second volume. Though he has 

 inserted a section on the "Explanation of Adaptation,""^' he says 

 nothing on Adaptations being the * ' definite results of the direct action 

 of changed conditions of life " (Darwin). 



He admits that the mutation theory " will explain adaptations just 

 as completely, or rather just as incompletely, .as the present view," by 

 which he means Darwinism ; yet he concludes by saying : * ' All the 

 difficulties . . . besetting the current view disappear if we substitute 

 mutability for fluctuating variability as the source of the origin of 

 species." Moreover, he has much to say upon the efiects of environ- 

 mental conditions in encouraging, or otherwise, abnormalities when they 

 have once put in an appearance. The capacity for making them may 

 lie dormant, as in the " atavists " in his experiments, and subsequently 

 reappear in the offspring. Professor de Vries does not appear to suggest any 

 cause for his idea of periodicity of mutations ; but it is at once explained 

 by the fact that variations only arise when new external conditions are 



* Pp. 606-14. 



