1899] NER VO US S J 'STEM IN ORGANIC E VOL UTION 2 6 1 



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 life. 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, i.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 I 



