684 



SGIENGE. 



[N. S. Vol. V. No. 122. 



isolated from the brain, salivary glands and 

 some other organs, there seems good reason 

 to believe that even these furnish to the 

 blood substances peculiar to them. 



It is, therefore, highly probable that 

 all organs have besides their obvious 

 function, a hidden function, in the 

 maintenance, by means of their internal 

 secretions, of the metabolic equilibrium of 

 the body. Further, many of these secre- 

 tions are absolutely essential to the life of 

 other organs, and in certain cases, as in the 

 thyroid and reproductive organs, they are 

 necessary to the development of organs ap- 

 parently not in any way connected with 

 them. There can be little doubt that one 

 of the prime uses of the blood is as a dis- 

 tributing agent of these substances, and 

 that its coordinative function is one of its 

 most important of&es. 



We are thus led to a possible explanation, 

 along these lines, of the organic unity of 

 organisms unprovided with a nervous sys- 

 tem. It is highly probable that the inter- 

 nal secretions play an important role in the 

 correlation of parts in the higher organisms. 

 It is possible that this role becomes the 

 principal one in case of the developing 

 embryo or of organisms like the plants 

 which have no nervous system. The inter- 

 nal secretions are also of interest in their 

 bearing on the correlation of variations. 



If an organ in one part of the organism 

 depends in any manner upon the internal 

 secretion of some other organ we may un- 

 derstand how the increased developement 

 of the one may lead to an increased de- 

 velopment of the other, though apparently 

 in no way connected with it. Thus we could 

 see how variations have arisen and how 

 they have been perpetuated until they are 

 themselves useful. Many organs, the be- 

 ginnings of which could hardly have been 

 useful enough to be acted upon by natural 

 selection, may have been developed because 

 they are correlated by means of their inter- 



nal secretions with other organs which are 

 useful. It would also be clear why certain 

 organs or groups of organs vary together. 

 If such organs are mutually interdepend- 

 ent in the manner indicated, then the 

 diminution of one necessarily means the 

 diminution of another and another and so 

 on. In certain cases the diminution of one 

 may lead to the growth of another organ. 

 This is, perhaps, most strikingly seen in 

 the case of castrated cattle, which are pro- 

 verbially large boned and fat, the growth 

 of the bone being correlated with the 

 diminution of the internal secretion of the 

 sexual organs. In another case, where two 

 organs were dependent on the internal 

 secretion of some third, the suppression of 

 one of the two might lead to a compensa- 

 tory growth in the other. 



That the internal secretions play a part 

 in embryonic differentiations seems very 

 probable. Striking examples of their im- 

 portance in the later stages of development 

 are afforded by the thyroid and the repro- 

 ductive glands, alreadjr referred to. Lack 

 of development of the thj^roid hinders the 

 development of the cranium and the whole 

 body. If cretins be fed on thyroid they in- 

 crease both in size and intelligence. The 

 development of the sexual oi'gans is essential 

 to that of many other so-called secondary 

 sexual characters. The same may very 

 well be the case in the embryo. Thus an 

 organ called into being by a previous organ 

 may in its turn determine, through an in- 

 internal secretion, the development of a 

 succeeding organ ; and we should here have 

 an explanation of the persistence of rudi- 

 ments, or the temporary appearance of 

 glands and organs which later disappear 

 and seem to fulfill no function whatever. 

 They may be necessary to the organism 

 through their internal secretions, which 

 give the necessary stimuli to the develop- 

 ment of other organs which are permanent.* 



*See note at end. 



