THEORIES OF HEREDITY 55 



We see that in this last hypothesis the inheritance of 

 acquired characters Darwin is at one with Spencer. Both 

 theories further agree in looking upon the germ-cells as 

 aggregations of particles derived from the body, both 

 necessarily assuming with this a circulation of these 

 particles through the body, and their migration towards 

 special points of the body, be they germ-cells or future 

 budding parts of it. 



In so far, the criticisms against Spencer's theory will hold 

 good also against Darwin. There is, however, a funda- 

 mental distinction between them. Spencer assumed his 

 physiological units to be all alike, their arrangement alone 

 leading to differentiation of organs, etc. : Darwin's gem- 

 mules, on the contrary, vary according to the special cells 

 they represent ; they can reproduce only those cells from 

 which they are derived. 



Valuable as the idea of specific representative particles has 

 proved to be for the study of heredity, it is, as propounded 

 by Darwin, hardly more than a formal solution of the 

 problem. As Weismann has put it : " If we suppose that 

 each cell arises from a special gemmule, and that these 

 gemmules are present whenever they are wanted, it is 

 easy to see how that structure, the origin of which we wish 

 to explain, may appear in any given position." Further- 

 more, to assume that the cells are all identical before they 

 attract the gemmules which give them their individuality, 

 and yet at the same time to invest these cells with the 

 most delicate selective power for the various gemmules 

 they attract, is, as Yves Delage has shown, not to solve the 

 question, but to put it anew. 



(c) GALTON'S STIRP THEORY. 



The lain objection against all pangenetic theories is the 

 assumption which has to be made, that the units or gem- 

 mules, or whatever their name may be, circulate freely 

 through the body, to accumulate finally in the sex-cells. 



