THEORIES OF PANGENESIS 409 



heritable," Galton supposed that " each cell may throw off a few 

 germs that find their way into the circulation, and have thereby 

 a chance of occasionally finding their way to the sexual elements 

 and of becoming naturalised among them." This part of his 

 theory is obviously a cautious admission of limited pangenesis 

 to account for a number of puzzling cases. 



Brooks' Theory. — In 1883, in his valuable work entitled The 

 Law of Heredity, Prof. W. K. Brooks gave full expression to 

 a modification of Darwin's view of pangenesis. The main 

 positions, which are here relevant, may be summarised as follows, 

 almost in the author's words : 



(i) The male and female cells are specialised in different 

 directions ; their union gives variability. 



(2) The ovum is a cell which has gradually acquired a compli- 



cated organisation, and which contains material particles 

 of some kind to correspond to each of the hereditary 

 characteristics of the species. 



(3) The ovum reproducing its like, as other cells, gives rise 



not only to the divergent cells of the organism, but also 

 to cells like itself. 



(4) Each cell of the body has the power to throw off minute 



germs. When, through a change in its environment, its 

 functions are disturbed, and its conditions of life become 

 unfavourable, it throws off small particles which are 

 the germs or gemmules of this particular cell. 



(5) These germs may be carried to all parts of the body. They 



may penetrate to an ovarian ovum or to a bud, but the 

 male cell has gradually acquired, as its especial and 

 distinctive function, a peculiar power to gather and 

 store up germs. 



(6) In fertilisation each gemmule unites with that particle of 



the ovum which is destined to give rise in the offspring 

 to the cell which corresponds to the one which produced 

 the gemmule, or else it unites with a closely related 



