76 THE THEORY OF NATURAL SELECTION 
stituent cells seem to be cut off from one another like 
so many closed boxes, it has been shown that there is 
almost universal communication between the proto- 
plasmic masses so enclosed, in the shape of minute 
fibrils of living substance which traverse the interven- 
ing walls. 
It would thus seem possible for liquid or easily 
soluble substances to pass freely from one part of the 
body of an organism to another. It is possible, for 
example, supposing the enlargement and strengthening 
due to the exercise of a particular muscle to be associ- 
ated with an increased production of some definite 
chemical substance, to imagine that an increased 
amount of the same substance might become enclosed in 
the germ-cells, so that this substance would be present 
in the offspring in greater abundance than would have 
been the case if the muscle of the parent had not been 
exercised. And this might facilitate a further develop- 
ment of the same muscle by exercise in the next 
generation. Ina similiar way increased bulk following 
upon better nutrition might be inherited, and this 
de Vries seems to have succeeded in showing to be 
actually the case in plants. Such changes might 
normally be so slight as to be almost imperceptible in 
a few generations, and yet after many generations 
might accumulate to an important extent. It would 
be impossible in practice to distinguish changes of this 
kind from what are known as accidental individual 
differences, and, indeed, there is no evidence at hand 
to disprove de Vries’ assertion that all continuous 
variations are of the nature of acquired characters— 
