RECAPITULATION 241 



knowledge now obtained concerning the nature and 

 action of hormones shows that such a process 

 actually exists, and in modern theory real substances 

 of the nature of special chemical compounds take 

 the place of the imaginary gemmules of Darwin's 

 theory of pangenesis or the ' constitutional units ' 

 of Spencer. 



7. The theory of the heredity of somatogenic modi- 

 fications by means of hormones harmonises with 

 and goes far to explain the facts of metamorphosis 

 and recapitulation in adaptive characters, and also 

 the origin of secondary sexual characters, their 

 correlation with the periodical changes in the gonads 

 and the effects of castration. At the same time 

 there are some somatic sex-characters, e.g. hi insects 

 and birds, which do not appear to be correlated with 

 changes in the gonads, and which are probably 

 gametogenic, not somatogenic in origin. 



8. The theory of the heredity of somatogenic 

 modifications is not in opposition to the mutation 

 theory. The author's view is that there are two 

 kinds of variation in evolution, one somatogenic and 

 due to external stimuli, acting either directly on 

 passive tissues or indirectly through function, and 

 the other gametogenic and due to changes in the 

 chromosomes of the gametes which are spontaneous 

 and not in any way due to modifications of the soma. 

 Adaptations are due to somatogenic modifications, 

 non-adaptive diagnostic characters to gametogenic 

 mutations. It is a mistake to attempt to explain 

 all the results of evolution by a single principle. 

 There are two kinds of congenital, constitutional or 

 hereditary characters in all organisms, namely, the 

 adaptive and the non-adaptive, and every distinct 



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