1899) WERVOUS SYSTEM IN ORGANIC EVOLUTION 261 
wrought by change in the environment, as otherwise heredity could 
never have any characters to work on. If that is not allowed, we 
must fall back on blind chance, or on the insinuation of some unknown 
power. Changes in environment can only be partial, since a complete 
change would destroy all organic hfe. But where changes are partial, 
and extending over vast periods of time, great changes may occur in 
the organism, as in the evolution of whales and seals from land 
animals. Now, where there are changes in environment leading to 
new habits in order to satisfy the vital functions, the organs or 
characters affected by the change of habit, being used in excess of their 
former use, are further developed, 7.e. their cellular elements are in- 
creased, either absolutely or relatively or both, since increase of use 
means increase of nutrition. But the cell activity is brought about, 
not directly, but indirectly, through the connection with the nerve- 
centres. Hence the increase of exercise in the nerve-centre leads to 
increase of nutrition, and this in turn to increased development of the 
nerve-centre. Thus, with the increase of function, there is also in- 
crease in size of the characters affected, and of the brain centres pre- 
siding over them. Increased use of a muscle leads to increase in size, 
and the brain centre of the muscles must also be changed in some 
way, for it too has done increased work. We know that the memory 
may be strengthened by exercise, and so with other special mental 
faculties. So too, as regards the special senses, the sailor’s eyesight is 
always better than the landsman’s. 
It is important, however, to remember that such changes take 
place chiefly in the young, and hence the importance of our conten- 
tion that the condition of the maternal body—cells, tissues, and organs 
affects the vitality of the developing ovum. The maternal con- 
ditions, acting as external stimuli to the ovum, must, as Weismann 
admits, affect the foetus, and I argue that they will produce such 
modifications as will bring the latter into harmony qualitatively and 
quantitatively with the maternal body. And as the general environ- 
ment reacts on the mother, and the mother on the embryo, it must be 
evident that the general environment has some influence on the 
developing germ or embryo. Now as the general environment of 
a mother in her successive production of offspring must vary, so too 
must the offspring vary. 
Assuming that the nervous system is to the fully-formed organism 
what the germ-plasm is to the ovum, we must see that there must be 
the same difference between the cells of the nervous system and the 
cells of the other portion of the organism as between germ-cells and 
somatic cells, for whereas the nervous system represents the whole 
body, a multum in parvo, and can induce the production of all kinds 
of cells, the somatic cells can only reproduce through the nervous 
system cells of their own kind. The egg-cell contains, as Naegeli says, 
all active specific characters as truly as the adult organism. What | 

