November 6, 1896.] 



SCIENCE, 



673 



grade, sub-grades and Phyla of the animal 

 kingdom evolution has been extremely slow 

 as compared with that in the higher. We 

 do not know the reason. It may be that 

 this remarkable persistence through the 

 stratified series of deposits is due to an in- 

 nate fixity of constitution which has rigidly 

 limited the power of variation; or, more 

 probably, perhaps, that the lower members 

 of the animal kingdom were, as they are 

 now, more closely confined to particular 

 environments, with particular sets of condi- 

 tions, with which they had to cope, and, 

 this being successfully accomplished, nat- 

 ural selection has done little more than 

 keep up a standard of organization which 

 was sufiicient for their needs ; while the 

 higher and more aggressive forms ranging 

 over many environments, and always prone 

 to encounter new sets of conditions, were 

 compelled to undergo responsive changes 

 or to succumb. But whatever be the 

 cause, the fact remains, and is of great 

 importance for our argument. When 

 the ancestor of one of the higher Phyla 

 was associated with the lower Phyla of 

 the Coelomate sub-grade, when further 

 back it passed through a Coelenterate, a 

 higher Protozoan, and finally a lower 

 Protozoan phase, we are led to believe that 

 its evolution was probably very slow 

 as compared with the rate which it subse- 

 quently attained. But this conclusion is 

 of the utmost importance ; for the history 

 contained in the stratified rocks nowhere 

 reveals to us the origin of a Phylum. And 

 this is not mere negative evidence, but 

 positive evidence of the most unmistakable 

 character. All the five Coelomate Phyla 

 which occur fossil appear low down in the 

 Palaeozoic rocks, in the Silurian or Cam- 

 brian strata, and they are represented by 

 forms which are very far from being primi- 

 tive, or, if primitive, are persistent types, 

 such as Chiton, which are now living. 

 Thus Vertebrata are represented by fishes, 



both sharks and ganoids ; the Appendic- 

 ulata by cockroaches, scorpions, Limu- 

 lids, Trilobites and many Crustacea; the 

 MoUusca by Nautilus and numerous allied 

 genera, by Dentalium, Chiton, Pteropods, 

 and many Gastropods and Lamellibranchs ; 

 the Gephyrea by very numerous Brachio- 

 pods and many Polyzoa ; the Echinoderma 

 by Crinoids, Cystoids, Blastoids, Asteroids, 

 Ophiuroids and Echinoids. It is just con- 

 ceivable, although, as I believe, most im- 

 probable, that the Vertebrate Phylum orig- 

 inated at the time when the earliest known 

 fossiliferous rocks were laid down. It must 

 be remembered, however, that an enormous 

 morphological interval separates the fishes 

 which appear in the Silurian strata from 

 the lower branches, grades and classes of 

 the Phylum in which Balanoglossus, the 

 Ascidians, Amphioxus and the Lampreys 

 are placed. The earliest Vertebrates to ap- 

 pear are, in fact, very advanced members 

 of the Phylum, and, from the point of view 

 of anatomy, much nearer to man than to 

 Amphioxus. If, however, we grant the 

 improbable contention that so highly or- 

 ganized an animal as a shark could be 

 evolved from the ancestral Vertebrate in the 

 period which intervened between the earli- 

 est Cambrian strata and the Upper Silurian, 

 it is quite impossible tO urge the same with 

 regard to the other Phyla. It has been 

 shown above that when these appear in the 

 Cambrian and Silurian they are flourishing 

 in full force, while their numerous special- 

 ized forms are a positive proof of a long 

 antecedent history within the limits of the 

 Phylum. 



If, however, we assume for the moment 

 that the Phyla began in the Cambrian, the 

 geologist's estimate must still be increased 

 considerably, and perhaps doubled, in order 

 to account for the evolution of the higher 

 Phyla from forms as low as many which are 

 now known upon the earth ; unless, indeed, 

 it is supposed, against the whole weight of 



