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 inôÙvesôÙtiôÙgaôÙtion, 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 proportions 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 says100 that by these chemical influences ãToute une partie de la construction des ûˆtres parait sãexpliquer dãune faûÏon toute mûˋcanique. La forteresse, si longtemps inaccessible, du vitalisme est entamûˋe. Car la notion morphogûˋnique ûˋtait, suivant le mot de Dastre101, comme ãle dernier rûˋduit de la force vitale.ããã
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 {137} all but sudden appearance on the earth of such exaggerated and almost monstrous forms as those of the great secondary reptiles and the great tertiary mammals102. We begin to see that it is in order to account, not for the appearance, but for the disappearance of such forms as these that natural selection must be invoked. And we then, I think, draw near to the conclusion that what is true of these is universally true, and that the great function of natural selection is not to originate, but to remove: donec ad interitum genus id natura redegit103.
The world of things living, like the world of things inanimate, grows of itself, and pursues its ceaseless course of creative evolution. It has room, wide but not