76 THE THEORY OF NATURAL SELECTION 



stituent cells seem to be cut off from one another like 

 so many closed boxes, it has beea 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. In a 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 



