ACQUIRED CHARACTERS DEFINED 123 



characters — peculiarities of any somatic cell which follow 

 from the structure of the original germ-cell — have there- 

 fore been called blastogenic by Weismann. They have 

 also been called spontaneous, because they spring up in 

 the individual without reference to the causes which 

 operate during its lifetime ; also inherent or centri- 

 fugal, because they belong to the essential nature of the 

 individual, and because they may be looked upon as 

 developing from within rather than as impressed from 

 without. Conversely, the characters which appear in 

 the somatic cells as the result of external influences, or 

 as the outcome of their own special or unusual activities, 

 — in fact, any characters appearing in the body which 

 were not predetermined in the original germ-cell, have 

 been called somatogenic, because their origin cannot be 

 traced to the structure of the original germ-cell, but is 

 entirely due to events brought about in somatic cells ; 

 they are also called acquired, because the individual comes 

 to possess them, although they do not belong to its 

 essential nature ; and centripetal, because they are im- 

 pressed upon the individual from without, and are not 

 the outcome of internal causes. 



It is my object to give a more detailed account of 

 these two theories of heredity, and then to allude very 

 briefly to some of the evidence which has been believed 

 to establish the hereditary transmission of acquired or 

 somatogenic characters. 



The first theory, maintaining that a close relationship 

 of a material kind exists throughout life between somatic 

 and germ-cells, was suggested by Darwin, under the 

 name of Pangenesis. 



This theory is illustrated by Diagram /, in which the 

 large circles, indicated by the capital letters P to W, 

 represent the body-cells of a Metazoon, which, for the 

 sake of simplicity, is supposed to be composed of only 

 sixteen somatic and four germ-cells, the latter being 

 placed in the centre. The somatic cells are arranged in 

 pairs, P P, QQ, &c, in order to indicate the fact that 

 similar cells are generally found on opposite sides of the 

 body in the higher Metazoa (bilateral symmetry). 



