136 THE RATE OF GROWTH [ch. 



proper to the male, as in certain extreme cases, known from time 

 immemorial, when late in life a hen assumes the plumage of the 

 cock. 



There are some very remarkable experiments by Gudernatsch, 

 in which he has shewn that by feeding tadpoles (whether of frogs 

 or toads) on thyroid gland substance, their legs may be made to 

 grow out at any time, days or weeks before the normal date of 

 their appearance*. No other organic food was found to produce 

 the same effect ; but since the thyroid gland is known to contain 

 iodine f , Morse experimented with this latter substance, and found 

 that if the tadpoles were fed with iodised amino-acids the legs 

 developed precociously, just as when the thyroid gland itself was 

 used. We may take it, then, as an established fact, whose full 

 extent and bearings are still awaiting investigation, that there 

 exist substances both within and without the organism which 

 have a marvellous power of accelerating growth, and of doing so 

 in such a way as to affect not only the size but the form or pro- 

 portions of the organism. 



If we once admit, as we are now bound to do, the existence 

 of such factors as these, which, by their physiological activity 

 and apart from any direct action of the nervous system, tend 

 towards the acceleration of growth and consequent modification 

 of form, we are led into wide fields of speculation by an easy and 

 a legitimate pathway. Professor Gley carries such speculations 

 a long, long way : for he says J that by these chemical influences 

 "Toute une partie de la construction des etres parait s'expliquer 

 d'une fa^on toute mecanique. La forteresse, si longtemps inacces- 

 sible, du vitalisme est entamee. Car la notion morphogenique 

 etait, suivant le mot de Dastre§, comme 'le dernier reduit de la 

 force vitale.' " 



The physiological speculations we need not discuss : but, to 

 take a single example from morphology, we begin to understand 

 the possibility, and to comprehend the probable meaning, of the 



* Cf. Loeb, Science, May 14, 191.5, 



f Cf. Baumann u. Roos, Vorkommen von lod im Thierkorper, Zeitschr. fur 

 Physiol. Chem. xxi, xxii, 1895, 6. 



% Le Neo- Vitalisme, Rev. Scientifique, Mars 1911, p. 22 (of reprint). 

 § La vie et la mort, p. 43, 1902. 



