Heredity. 145 



In 1883, in his valuable work entitled The Law of 

 Heredity, Professor W. K. Brooks gave full expression 

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

 main positions, which are here relevant, may be sum- 

 marized as follows, almost in the author's words: 



(1) The male and female cells are specialized in different 

 directions ; their union gives variability. 



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

 plicated organization, 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 do, gives rise 

 not only to the divergent cells which build up the body of the 

 organism, but also to cells like itself, which are the future repro- 

 ductive cells. 



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

 germs. The cell does this especially when some change in its 

 environment has disturbed its functions. 



(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 fertilization each germ or gemmule unites with that 

 particle of the ovum which is destined to give rise in the off- 

 spring to the cell which corresponds to the one which produced 

 the gemmule, or else it unites with a closely-related particle, 

 destined to give rise to a closely-related cell. Such a cell will 

 be a hybrid, tending to vary. 



(7) As the ovarian ova of the offspring share by direct in- 

 heritance all the properties of the fertilized ovum, the organisms 

 to which they give rise will tend to vary in the same way. 



(8) A cell which has thus varied will continue to throw off 

 gemmules, and thus to transmit variability to the corresponding 

 part in the bodies of successive generations of descendants, 

 until a favourable variation is seized upon by natural selection. 



(9) As the ovum which produced this selected organism will 

 transmit the same variation to its ovarian ova by direct inherit- 

 ance, the characteristic will be established as specific, and 

 transmitted henceforth without gemmules. 



The above theory, being important, has been stated 

 at some length. Apart from the suggestion of vari- 

 ation as due to sexual intermingling, with which Weis- 

 mann has made us more familiar; apart, too, from the 

 suggestion of germinal continuity, the credit of which 



( M 623 ) K 



