xxi EVOLUTION OF EVOLUTION THEORIES 427 



supposed. The work of Bateson and of De Vries has 

 shown that changes of considerable magnitude may cccur 

 at a single leap. When the new character that appears 

 suddenly has a considerable degree of perfection from 

 the moment of its emergence, is independently heritable, 

 and does not blend, it is called a mutation. (5) These 

 mutations are not liable to be swamped by intercrossing, 

 as Darwin supposed. They have great staying power 

 in inheritance, reappearing persistently in a certain 

 proportion of the descendants. (6) Structural changes 

 directly induced in the life-time of the individual by some 

 peculiarity in environment or in function are distinguished 

 as somatic modifications, or, unfortunately, as " acquired 

 characters." Variations are endogenous or blastogenic, 

 arising from a change in the germ-plasm ; modifications 

 are exogenous or somatogenic, impressed from without. 

 There is no convincing evidence that modifications are 

 as such or in any representative degree transmissible. 

 (7) As to the origin of variations, we must still confess 

 with Darwin that " our ignorance of the laws of variation 

 is profound," but certain possibilities have become clearer. 

 Thus certain kinds of variation are interpretable as due 

 to a dropping out of some constituent or factor in the 

 inheritance, others as due to an augmentation of a factor ; 

 others as due to a novel arrangement or pattern of factors 

 which were present in the ancestry ; and there are in the 

 history of the germ-cells various opportunities (e.g. in 

 maturation and fertilisation) for such new permutations 

 and combinations. Then again it is possible that en- 

 vironmental stimuli operating from the outer world, or 

 fluctuations in the nutritive stream within the body may 

 induce changes in the complex germ-plasm. But as to 

 new departures of moment, it is not possible at present 

 to offer any explanation. They are expressions of a 

 primary quality of living organisms for the germ-cell 

 is an implicit organism. They are manifestation of a 

 capacity for creative evolution. They seem to us like 

 experiments in self-expression. 



In regard to heredity, the scientific study of which may 

 be said to have been inaugurated by Darwin, some notable 



