But the more primitive chemical bond persists, and is scarcely 

 diminished in importance, but only over-shadowed, by the more 

 easily recognisable reactions due to the working of the nervous 

 system. In higher animals we may recognise special chemical 

 means whereby chemical coadaptations are established and main- 

 tained at a normal level or in certain circumstances altered. These 

 are the internal secretions produced by sundry organs, whether by 

 typical secretory glands (in which case the internal secretion is 

 something additional and different from the external secretion), or 

 by the so-called ductless glands, such as the thyroid, the thymus, 

 the adrenal bodies, or by organs which cannot strictly be called 

 glands, namely, the ovaries and testes. All these produce chemical 

 substances which, passing into the blood or lymph, are distributed 

 through the system, and have the peculiar property of regulating 

 or exciting the specific functions of other organs. Not, however, 

 of all the organs, for the different internal secretions are more or 

 less limited and local in their effects, one affecting the activity of 

 this and another the activity of that kind of tissue or organ. 

 Starling proposed the name hormones for the internal secretions 

 because of their excitatory properties (op*au), to stir up, to excite). 

 " Hormones have been studied chiefly from the point of view 

 of their stimulating effect on the metabolism of various organs. 

 From the morphologist's point of view, interest chiefly attaches to 

 the possibility of their regulating and promoting the production of 

 form. It might be expected that they should be efficient agents 

 in regulating form, for, if changes in structure are the result of 

 the activities of groups of cells, and the activities of cells are the 

 results of the activities of the enzymes which they contain, and if 

 the activities of the enzymes are regulated by the hormones, it 

 follows that the last-named must be the ultimate agents in the 

 production of form. It is difficult to obtain distinct evidence of this 

 agency but in some cases, at least, the evidence is sufficiently clear. 

 " I will confine myself to the effects of the hormones produced 

 by the testes and ovaries. These have been proved to be intimately 

 connected with the development of secondary sexual characters, 

 such, for instance, as the characteristic, shape and size of the horns 

 of the bull; the comb, wattles, spurs, plumage colour in poultry; 

 the swelling on the index finger of the male frog; the shape and 

 size of the abdominal segments of crabs. These are essentially 

 morphological characters, the results of increased local activity of 

 cell-growth and differentiation. As they are attributable to the 



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