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 oQ 
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 
