38 WILLIAM KEITH BROOKS 



In developing this idea into an explanatory theory of the way 

 in which hereditary transmission is accomplished, Brooks borrows 

 from Darwin's hypothesis of pangenesis, and assumes , the exis- 

 tence of material particles, "gemmules," which are thrown off 

 from the body cells. Unlike Darwin, however, he assumes that 

 such particles are only thrown off at particular periods, when the 

 body cells are disturbed in function through some change in their 

 environment. The gemmules may penetrate an ovum or a bud, 

 but it is the male germ cell which has gradually acquired during 

 the evolution of the metazoa the peculiar power to gather and 

 store up gemmules. The ovum on the other hand has acquired 

 a very different nature. It contains material particles which 

 correspond to the hereditary characteristics of the species. Thus 

 in the case of a fertilized egg, as in that of a parthenogenetic egg, 

 the great bulk of the development is due to the properties of the 

 ovum itself. The gemmules brought in by the sperm cell unite 

 with homologous particles in the ovum and so composite particles 

 are produced which, as the egg segments and develops, give rise 

 to cells that are strictly hybrids and which therefore exhibit 

 variation. The ovum thus is the conservative element which 

 transmits the characteristics that have already been acquired. 

 The male cell is peculiarly that which stores up the disturbing 

 effects of a changing environment. It especially leads, therefore, 

 to variability in the offspring, to the production of individual 

 differences. 



This ingenious hypothesis enables Brooks to explain a great 

 variety of inheritance phenomena and to overcome several ser- 

 ious objections to the unassisted selection theory. Whatever 

 truth there may or may not be in the special ideas of the book, it 

 remains to-day a stimulating and suggestive contribution, and it 

 is properly looked on as one of the factors that have in recent 

 years focussed the attention of the biological world on the prob- 

 lems of heredity. 



Minor papers dealing with heredity and evolution, the causes 

 of variation, and the determination of sex, appeared from time to 

 time. Sections of the " Foundations of Zoology " (1899) show, too, 

 that Brooks' interest in the questions discussed in the "Law of 



