Theories of Evolution 29 



defined, and as pure of type as might be ex- 

 pected of any species. 



These new species are not produced once or 

 in single individuals, but yearly and in large 

 numbers. The whole phenomenon conveys the 

 idea of a close group of mutations, all belonging 

 to one single condition of mutability. Of course 

 this mutable state must have had a beginning, 

 as it must sometime come to an end. It is to 

 be considered as a period within the life-time of 

 the species, and probably it is only a small part 

 of it. 



The detailed description of this experiment, 

 however, I must delay to a subsequent lecture, 

 and I may be allowed to state, that the discov- 

 ery of this period of mutability is of a definite 

 theoretical importance. One of the greatest ob- 

 jections to the Darwinian theory of descent 

 arose from the length of time it would require 

 it* all evolution was to be explained on the 

 ground of slow and nearly invisible changes. 

 This difficulty, is at once met, and fully sur- 

 mounted by the hypothesis of periodical but 

 sudden and quite noticeable steps. This as- 

 sumption requires only a limited number of 

 mutative periods, which might well occur within 

 the time allowed by physicists and geologists 

 for the existence of animal and vegetable life 

 on the earth. 



