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-tune 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 

 if 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. 



