io68 The American Naturalist, [December, 
the fundamental characters of mammalian dentition were laid dur- 
ing adolescence, since in the primitive types the temporary denti- 
tion was nearly wanting-. The tritubercular molar was estab- 
lished at that time, and owes its present existence to inheritance. 
Only the sectorial and lophodont types have been added since the 
extensive development of the milk dentition in geologic time. 
The preceding statements do not of course constitute an ex- 
planation of the exact manner in which a stimulus which effects 
say the contraction of a muscle, effects molecular movements of 
the nuclei of the reproductive cells. This is a question of organic 
molecular physics ; a science which has made scarcely a fcegin- 
ning. That the transmission of such influence is primarily through 
the nervous system and secondarily through nutrition, may be 
safely assumed. That the modus operandi is similar to that 
which produces reflexes may be also reasonably supposed. How 
the records of these movements become reflexes, is concentrated in 
a reproductive cell, is a question to be solved only in a more ad- 
vanced stage of knowledge of organic physics than we now pos- 
Speculation in this direction takes the following forms. The 
energy or molecular movement must be transmitted to the germ- 
plasma through a material or molecular basis. This basis, it may 
be supposed, must be that which receives the mechanical im- 
pression which is to produce a corresponding modification of 
growth energy in the ovum or spermatozooid ; that is, in the case 
of a modified bone articulation, particles of matter must pass from 
the latter through the medium of the circulation to the reproduc- 
tive cells. The alternative hypothesis is, that the nervous energy 
(neurism) which directs the active region to make or omit to 
make a given movement, the result of which is to be structural 
modification in the young, is impressed through nervous chan- 
nels, on the germ-cells of either sex. In this case the transmis- 
sion of particles of matter is not necessary, as material connection 
through the nervous threads already exists. 
To the first of these points of view belong the pangenesis 
theory of Darwin, and the modified pangenesis of Brooks.^ These 
1 The I^w of Heredity, Baltimore, 1883, p. 80. 
