November 27, 1885.] 



SCIENCE. 



481 



fact : it is determined by natural conditions, and 

 not by the voluntary decision of individuals. 



J.J. 



A SUGGESTION FROM MODERN EMBRY- 

 OLOGY. 



One of the obstacles which proved to be a diffi- 

 culty of considerable weight to Darwin in his 

 appUcation of the descent theory was the sudden 

 appearance of a highly developed fauna in the 

 Silurian age. Tliis difficulty has not decreased, 

 but has rather increased with the further knowl- 

 edge of that fauna. The primordial fauna, as 

 shown by the fossils of the Silurian rocks, was not 

 what naturalists would have assumed had they 

 been called upon to construct this fauna from 

 a priori grounds. Instead of a few simple gener- 

 ahzed forms, these early rocks showed evidence of 

 a higlily diversified fauna. In the Silurian rocks 

 are represented aU of the great divisions of the 

 animal kingdom, including even the vertebrates. 

 Moreover, of the smaller divisions, a sufficient 

 number are here represented to cause considerable 

 surprise. About five-sixths of the orders now 

 existing, nearly an equal proportion of sub-orders, 

 a great many families and some genera of to-day 

 are found in these earhest rocks. It is indeed 

 remarkable to find such a very large number of 

 existing groups represented in the earliest fauna 

 of which we have any knowledge. It is true that 

 the Silurian age lasted a long time, and that in 

 the lower Silurian the fauna is not quite so diverse 

 as above indicated ; but even here it is sufficiently 

 diverse to be surprising. When the history of 

 vertebrates since that time is compared with the 

 history of other groups, the contrast is very strik- 

 ing. They have had time enough to develop from 

 the very lowest forms — which we judge lived in 

 the Silurian times — into the present highly diver- 

 sified gTOups. But with all other groups of ani- 

 mals the advance has been comparatively small. 

 It must be assumed, to reconcile these facts with 

 evolution, that enough time elapsed between the 

 beginning of life on the world and the beginning 

 of the Silurian to develop aU of the sub-kingdoms 

 except the vertebrates to a high degree of differ- 

 entiation. And, when the great amount of time 

 which it has required to develop the vertebrates is 

 taken into consideration, the amount of lost time 

 necessary to assume previous to the Silurian seems 

 too great to be credible. 



It wiU, of course, never be possible to reconcile 

 the Silurian fauna with evolution without the 

 assumfjtion of a long lost period of this character. 

 But certain general results from modern embry- 

 ology are in this connection suggestive, and indi- 



cate that the difficulty is not so great as has been 

 sometimes conceived. For modem embryology is 

 teaching us that our various sub-kingdoms are all 

 direct modifications of the most primitive multi- 

 cellular animal. Using embryology as a guide in 

 interpreting animal history, naturalists have been 

 continually shortening this history, particularly at 

 the bottom. From the time when Haeckel traced 

 the genealogy of man through twenty-one stages, 

 these stages have one by one been dropped by 

 naturalists, with the result of making the history 

 a much more direct one. Finally, the recent the- 

 ories of Sedgwick, and others who follow him 

 wholly or partially, would make the history of aU 

 animals much shorter by showing that all the sub- 

 kingdoms may be regarded as resulting directly 

 from modifications of the gastrula by slight changes 

 in its shape. We once derived the worms from the 

 coelenterates, the annelids from the lower worms, 

 and the vertebrates from the annelids ; but now- 

 all of these groups are derived directly from the 

 gastrula itself. This theory of Sedgwick is re- 

 ceiving support in some form from many sources — 

 at least, so far as concerns this feature of it. There 

 is certainly a tendency to-day to look upon a 

 greater and greater number of types as direct mod- 

 ifications of the original animal represented by the 

 gastrula stage. Coelenterates, polyzoa, brachi- 

 opods, moUusks, annelids, and vertebrates have 

 all been shown to be derivable from the gasti-ula 

 by simple direct modifications. 



Now, we must remember that slight variations 

 at the bottom of a diverging series produce much 

 greater effects than variations higher up. AVhen 

 a tree is first sprouting, differences in the direction 

 of its buds determine the shape of the future ti'ee ; 

 for these early buds become the great branches, 

 and the slightest difference in their du-ection is 

 enough to cause a wide separation between them 

 as growth goes on. After the tree has grown to a 

 considerable size, its buds no longer produce great 

 branches, but only small ones, or perhaps only 

 twigs. Growth cannot now change the general 

 shape of the tree, but only increase the profusion 

 of small branches, twigs, and leaves. That such a 

 relation represents the history of the various groups 

 of the animal kingdom is unquestionably the 

 teaching of modern embryology. 



The significance of this result in enabhng us to 

 understand the fauna of the Silui'ian rocks is 

 evident enough. It not only shortens the time 

 necessary to be assumed prior to the Silurian, but 

 it also enables us, partially at least, to imderstand 

 the presence at this early period of such a lai'ge 

 number of our present existing types. For the 

 protozoan to develop into the first multicellular 

 animal, represented by the gastrula, must have 



