September 17, 1920] 



SCIENCE 



261 



not in the paleontologist, if we regard them as 

 erratic pioneer tracks blazed througli a tangled 

 forest. As our acquaintance with fossils en- 

 larged, the general direction became clearer, 

 and certain paths were seen to be impossible. 

 In 1881, addressing this association at York, 

 Huxley could say: 



Fifty years hence, whoever undertakes to record 

 the progress of paleontology will note the present 

 time as the epoch in which the law of succession 

 of the forms of the higher animals was determined 

 by the observation of paleontological facts. He 

 will point out that, just as Steno and as Cuvier 

 were enabled from their knowledge of the em- 

 pirical laws of coexistence of the parts of ani- 

 mals to conclude from a part to a whole, so the 

 knowledge of the law of succession of forms em- 

 powered their successors to conclude, from one or 

 two terms of such a succession, to the whole series, 

 and thus to divine the existence of forms of life, 

 of which, perhaps, no trace remains, at epochs of 

 inconceivable remoteness in the past. 



DESCENT NOT A COROLLARY OF SUCCESSION 



!N"ote that Huxley spoke of succession, not of 

 descent. Succession undoubtedly was recog- 

 nized, but the relation between the terms of the 

 succession was little understood, and there was 

 no proof of descent. Let us suppose all written 

 records to be swept away, and an attempt made 

 to reconstruct English history from coins. We 

 could set out our monarchs in true order, and 

 we might suspect that the throne was heredi- 

 tary; but if on that assumption we were to 

 make James I. the son of Elizabeth — well, but 

 that's just what paleontologists are constantly 

 doing. The famous diagram of the Evolution 

 of the Horse which Huxley used in his Ameri- 

 can lectures has had to be corrected in the 

 light of the fuller evidence recently tabulated 

 in a handsome volume by Professor H. F. Os- 

 born and his coadjutors. Palwoiherium, 

 which Huxley regarded as a direct ancestor of 

 the horse, is now held to be only a collateral, 

 ,as the last of the Tudors were collateral an- 

 cestors of the Stuarts. The later Anchitherium 

 must be eliminated from the true line as a 

 side-branch— a Young Pretender. Sometimes 

 an apparent succession is due to immigration 

 of a distant relative from some other region — 



" The glorious House of Hanover and Prot- 

 estant Succession." It was, you will re- 

 member, by such migrations that Ouvier ex- 

 plained the renewal of life when a previous 

 fauna had become extinct. He admitted suc- 

 cession but not descent. If he rejected special 

 creation, he did not accept evolution. 



Descent, then, is not a corollary of suc- 

 cession. Or, to broaden the statement, his- 

 tory is not the same as evolution. History is 

 a succession of events. Evolution means that 

 each event has sprung from the preceding 

 one. ISTot that the preceding event was the 

 active caiise of its successor, but that it was 

 a necessary condition of it. For the evolu- 

 tionary biologist, a species contains in itself 

 and its environment the possibility of pro- 

 ducing its successor. The words " its envir- 

 onment " are necessary, because a living 

 organism can not be conceived apart from its 

 environment. They are important, because 

 they exclude from the idea of organic evolu- 

 tion the hypothesis that all subsequent forms 

 were implicit in the primordial protoplast 

 alone, and were manifested either through a 

 series of degradations, as when thorium by 

 successive disintegrations transmutes itself to 

 lead, or through fresh developments due to 

 the successive loss of inhibiting factors. I 

 say " a species contains the possibility " rather 

 than " the potentiality," because we can not 

 start by assuming any kind of innate power. 



Huxley, then, forty years ago, claimed that 

 paleontologists had proved an orderly suc- 

 cession. To-day we claim to have proved 

 evolution by descent. But how do we prove 

 it? The neontologist, for all his experi- 

 mental breeding, has scarcely demonstrated 

 the transmutation of a species. The paleon- 

 tologist can not assist at even a single birth. 

 The evidence remains circumstantial. 



RECAPITULATION AS PROOF OF DESCENT 



Circumstantial evidence is convincing only 

 if inexplicable on any other admissible theory. 

 Such evidence is, I believe, afforded by paleon- 

 tological instances of Haeckel's law — i. e., the 

 recapitulation by an individual during its 

 grovTth of stages attained by adults in the 



