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 limitation of the 
supply of oxygen) such extensive chemical changes that morphological 
changes of the tissue ensue. If this is really the case, a possibility 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 metabolism).1 
This quotation is in harmony with the suggestion of Professor 
Wilson as to the operation of “‘ hormones.” 2 
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. 2 Ibid., p. 79. 
3 Cf. Conn, Method of Evolution, pp. 108 ff.; Wallace, Darwinism, ch. III. 
4 Thomson mentions three classes of struggle for existence: (1) 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.” 
