236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



bryonic to the adult state. As with phylogeny — ^the history of the 

 race — a whole series of addresses would be necessary to exploit this 

 theme adequately. Hence, I must be content with but two illustra- 

 tions confined to postembryonic development, by way of illuminating, 

 if possible, the trend of my contentions in this address. 



According to the recapitulation theory, with which I have no 

 quarrel, every animal climbs its own ancestral tree. That is to say, 

 it repeats in the course of its development, from the embryonic to 

 the adult stage, the successive stages which marked the development 

 of its ancestors, near and remote. Many objections have been raised 

 to this conception, but these were largely based on a too narrow 

 interpretation of its terms. 



In its essential features there can be no escape from "recapitula- 

 tion." For though all living organisms, plant or animal, are com- 

 posed of the same basic substances, so far as we can analyze them, 

 in no two are the qualities of that substance alike; as I showed, in 

 the beginning of this address, in the cases of the mold Penicillium; 

 and the qualities of the tissues of the ox and the sheep, or the grouse 

 and the pheasant. Hence, they will respond differently to similar 

 stimuli. The effects of such stimuli are found in changes of form 

 in adjustment to "use." Whether the organism be a single cell, or a 

 vast complex of cells, it will retain and intensify its responses to 

 such stimuli after its own peculiar fashion, so long as its race exists. 

 That is to say its responses or "acquirements" are transmitted from 

 one generation to another. This conclusion is inevitable. But it is 

 by no means in accordance with current views inspired by Weiss- 

 mann's germ-plasm theory. 



According to this there are two kinds of "plasm" — the "germ- 

 plasm" and the "somato-plasm." This last forms the visible, tangible 

 body which is derived from the germ-plasm leading a cloistered life 

 within the body it has given rise to, but not of it. It is held, then, 

 that it is the germ-plasm which forms the body ; and any differences 

 observable between any two closely related bodies, whether of the 

 same species or more distantly related, are due entirely to changes 

 which have come about in that germ-plasm, which is supposed to be 

 unstable, and thus constantly giving rise to the variations we find 

 expressed in the somato-plasm. The germ-plasm is the golden calf 

 of biology, and woe betide those who fail to render it homage. 



Let me take two concrete cases of "recapitulation." Twenty would 

 serve me better. They are shown on plate 6. 



The first is that of the postembryonic stages of the angler fish, 

 chosen because of its striking contrasts. In the earliest stage it will 

 be noted the body tapers backward to a point. Of the fins there is 

 but a small pectoral, a rudimentary first dorsal fin ray and pelvic 



