256 



SCIENCE. 



[N. S. Vol. VIII. No. 191. 



brian animal known to us was the Ortho- 

 ceratites, but its remains have not yet 

 been detected below the 2d Cambrian zone. 

 Even if some protocordate Balanoglossus, 

 Ascidians or even Amphioxus had already 

 begun their existence in these Precambrian 

 times they could have caused but a little 

 more destruction of life than their contem- 

 poraneous invertebrate allies. As the re- 

 mains of Ostracodermi and sharks have 

 been detected in Trenton strata, perhaps 

 they originated in the Cambrian, when they 

 must have been active forces in the elimina- 

 tion of those Precambrian soft-bodied ani- 

 mals which connected classes now quite 

 wide apart. 



The rapid increase in the Precambrian 

 population was hastened by the probable 

 fact that this, more than any subsequent 

 period, was one of rapid migration and 

 colonization. Vast areas of the shallow 

 depths over the site of the embryo con- 

 tinent, more or less shut off from the main 

 ocean by shoals, reefs and islands, were, by 

 oscillations of the sea-bottom and land, 

 opened up at various times to migrants 

 from the older previously settled seas. 



The nature of the Precambrian sediments 

 shows that the more open sea-bottom was 

 swept by tidal and ocean currents varying 

 in strength and extent. The topography 

 of the ocean bottom over what is now land 

 must have been more diversified than at 

 present. In the late ages of the Algonkian, 

 owing to active competition and the strug- 

 gle for existence in the overstocked areas, 

 the process of segregation or geographical 

 isolation was rapidly effected, and the mi- 

 grants from the denser centers of growth 

 pressed into the then uninhabited areas 

 where, as new, vigorous and prepotent 

 colonists, they broke ground and founded 

 new dynasties. 



At such times as these we can easily 

 imagine that, besides the absence of com- 

 petition, the Lamarckian factors of change 



of surroundings bringing about new habits 

 and thus inducing new needs, the use and 

 disuse of organs, together with the inherit- 

 ance of characters acquired during the life- 

 time of the individual, operated then far 

 more rapidly and in a much more thorough- 

 going way than at any period since, while 

 all through this critical, creative period, as 

 soon as there was a sufficient diversity in 

 the incipient forms and structures, a selec- 

 tive principle began to operate. 



For forty years past, since the time of 

 Darwin, the idea that these early forms 

 were more rapidly evolved, and that they 

 were more plastic than forms now existing, 

 has constantly cropped out in the writings 

 of our more thoughtful and studious pale- 

 ontologists and biologists. 



Darwin, in his Origin of Species, as 

 quoted by Walcott with approval, remarked 

 that it is indisputable that, before the lowest 

 Cambrian stratum was deposited, long 

 periods elapsed, as long as, or probably far 

 longer than, the whole interval from the 

 Cambrian age to the present day ; and that 

 ' during these vast periods the world 

 swarmed with living creatures.' Darwin 

 then adds : " It is, however, probable, as 

 Sir William Thompson insists, that the 

 world at a very early period was subjected 

 to more rapid and violent changes in its 

 physical conditions than those now occur- 

 ring ; and such changes would have tended 

 to induce changes at a corresponding rate 

 in the organism which then existed." 



Professor Hyatt,* from his exhaustive 

 studies on the Nautiloidea and Ammo- 

 noidea, concludes : 



" These groups originated suddenly and 

 spread out with great rapidity, and in some 

 cases, as in the Arietidje of the Lower Lias, 

 are traceable to an origin in one well-defined 

 species, which occurs in close proximity to 

 the whole group in the lowest bed of the 



*Phylogeny of an Acquired Characteristic. Proc> 

 Amer. Phil. Soc, XXXII., p. 371. 



