The Hypothesis of Periodic Mutability. 205 



with this theory. And the fact that ordinary or collective 

 species consist of groups of elementary species whose 

 characters may differ in every conceivable zvay empha- 

 sises the existence of indiscriminate mutability. 



§ 27. THE HYPOTHESIS OF PERIODIC MUTABILITY. 



Tlie constancy of species is a demonstrated fact ; 

 their transmutabihty is still a matter of theory. This 

 is the old objection against the theory of descent. La- 

 marck, Darwin and Wallace met this difficulty by 

 assuming that the immutability was only apparent and 

 was due to the fact that the changes are so slow that in 

 the short time during which we are able to observe them 

 they can not be detected. 



This however is merely an assumption, as I have al- 

 ready shown. No one doubts that many species have 

 undergone vast changes during the course of centuries ; 

 but no one knows whether they have taken place grad- 

 ually or by leaps and bounds. 



The contrary supposition that species can remain ab- 

 solutely unchanged during long periods of time but under 

 certain circumstances begin to produce new forms seems 

 to me at least equally justified. The ancestors of species 

 that exist to-day have on this theory passed through im- 

 mutal)le and mutable periods : the division of the large 

 species into elementary species would be the result of the 

 last or of some of the last periods of mutability.^ 



We repeatedly find the idea of a periodic transmuta- 

 tion of species expressed in Darw^in's works. "Changed 



^ KoLLMANN remarks on this subject : 'Tn no species of animal 

 or plant is this process — the formation of new races — a perpetual 

 one but is confined to certain periods. If this were not the case we 

 should have only changing forms and always new species instead 



