Untitledinternal secretion, such as the thyroid and the supra-renal. Thesemanufacture subtle chemical substances which are distributed by theblood throughout the body, and have a manifold influence in regulatingand harmonising the vital processes. Some of these chemical messengersare called hormones, which stimulate organs and tissues to greateractivity; others are called chalones, which put on a brake. Someregulate growth and others rapidly alter the pressure and compositionof the blood. Some of them call into active development certain parts ofthe body which have been, as it were, waiting for an appropriatetrigger-pulling. Thus, at the proper time, the milk-glands of amammalian mother are awakened from their dormancy. This very interestingoutcome of evolution will be dealt with in another portion of this work.THE INCLINED PLANE OF ANIMAL BEHAVIOUR§ 1Before passing to a connected story of the gradual emergence of higher
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