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gurely chemical means. In protozoa, because of their small size, the question 

 of coadaptation of function hardly comes into question; but in sponges, many of 

 which are of large size, the mechanism of coadaptation must also be almost exclu- 

 sively chemical. Thus we learn that the simplest and, by inference, the phyle- 

 tically oldest mechanism of reaction and co-ordination is a chemical mechanism. 

 In higher animals the necessity for rapid reaction to external and internal stimuli 

 has led to the development of a central and peripheral nervous system, and as we 

 ascend the scale of organisation, this assumes a greater and greater importance as 

 a co-ordinating bond between the various organs and tissues of the body. But 

 the more primitive chemical bond persists, and is scarcely diminished in im- 

 portance, but only overshadowed 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 maintained at 

 a normal level, or under 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. Alt 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 then 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 

 [opfxdoi, to stir up, to excite). 



Hormones have been studied chiefly from the point of view of their stimu- 

 lating effect on the metabolism of various organs. From the morphologist's point 

 of view, interest chiefly attaches to the possibility of their regulating and pro- 

 moting 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 1 — such, for instance, as the characteristic shape and 

 size of the horns of the bull; the comb, wattles, spurs, plumage colour, and spurs 

 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 stimulating effect of the hormone produced by the male 

 organ in each species, they afford at least one good instance of the production 

 of a specific change of form as the result of an internal chemical stimulus. We 

 get here a hint as to the nature of the chemical mechanism which excites and 

 correlates form and function in higher organisms; and, from what has just been 

 aaid, we perceive that this is the most primitive of all the animal mechanisms. I 

 submit that this is a step towards forming a clear and concrete idea of the inner 

 nature of the organism. There is one point, and that a very important one, upon 

 which we are by no means clear. We do not know how far the hormones them- 

 selves are liable to change, whether by the action of external conditions or by the 

 reciprocal action of the activities of the organs to which they are related. It is at 

 least conceivable that agencies which produce chemical disturbances in the cir- 

 culating fluids may alter the chemical constitution of the hormones, and thus 

 produce far-reachng effects. The pathology of the thyroid gland gives some 

 ground for belief that such changes may be produced by the action of external 

 conditions. But, however this may be, the line of reasoning that we have followed 



1 .See J. T. Cunningham, " The Heredity of Secondary Sexual Characters in 

 relation to Hormones, a Theory of the Inheritance of Somatogenic Characters." 

 Archiv f. Entwicklungsmechanik, xxvi., 1908. 



