76 ADAPTATION AND PROGRESS 



as it can affect the chemical processes of the body. As illustrations of a 

 chemical effect of the nerves, the fact is mentioned that stimulation of the 

 nerves of certain glands produces a secretion. Mathews has shown, however, 

 that in cases where stimulation of the sympathetic system produces a secre- 

 tion, the glands contain muscular fibres which contract when stimulated, 

 and in this way press a liquid out of the ducts. . . . There are no specifi- 

 cally trophic nerves, but it is possible that many nerves produce indirectly 

 (for instance, through disturbances of the circulation and Kmitation of the 

 supply of oxygen) such extensive chemical changes that morphological 

 changes of the tissue ensue. If this is really the case, a possibihty still exists 

 that the central nervous system also affects the sexual cells indirectly, in so 

 far as disturbances of circulation and hence chemical changes are produced, 

 which may modify the sexual cells contained in the testes and ovaries chemi- 

 cally. Thus there might be a very remote chance that brain-activity of one 

 generation might lead to the formation of chemical substances which affect 

 the sexual cells. . . . We arrive thus at the conclusion that a transmission 

 of hereditary characteristics through the egg is only possible in the form of 

 specific chemical substances, and that the central nervous system could only 

 influence heredity, if it could bring about the formation of special substances 

 in the egg (by influencing metabohsm).i 



This quotation is in harmony with the suggestion of Professor 

 Wilson as to the operation of " hormones.'' ^ 



In the babel of voices can we hear a single clear word of use in 

 the study of social progress ? That nature is prodigal is certain 

 but decreasingly so as we rise in the scale to the higher species 

 where a large proportion of the offspring reach maturity. Varia- 

 tion is the law of life, — and more universal than Darwin 

 imagined.^ Struggle for existence is unquestionable if we accept 

 the term in the large and metaphorical sense as used by Darwin 

 and more recently by Thomson."* As to the causes of variation, 

 however, the " doctors disagree " so too, as to the potency of 



1 Physiology of the Brain, pp. 208 ff. ^ /J^.^ p. ^q. 



3 Cf. Conn, Method of Evolution, pp. 108 ff.; Wallace, Darwinism, ch. III. 



* Thomson mentions three classes of struggle for existence: (i) struggle between 

 fellows, (2) struggle between foes, (3) struggle with fate. In the first, " the struggle 

 does not need to be direct to be real, — the essential point is that the competitors 

 seek after the same desiderata of which there is a limited supply. In the second, 

 it is between individuals and between species, sometimes to the death. In the 

 third, our sweep widens still further, and we pass beyond the idea of competition 

 altogether, to cases where the struggle for existence is between the living organism ' 

 and the inanimate conditions of life, — for instance, between birds and the winter's 

 cold, between aquatic animals and changes in the water, between plants and 

 drought, between plants and frost ... in a wide sense, between Life and Fate." 



