TRANSACTIONS OP THE SECTIONS. 101 



extinct representatives. We are tlius enabled to lay down the further principle, 

 that the individual, in the course of its own development from the eg^ to the fully 

 formed state, recapitulates within that short period of time the various forms which 

 its ancestry presented in consecutive epochs of the world's history ; so that if we 

 knew all the stages of its individual development, we should have a key to the 

 long line of its descent. Through the hypothesis of Evolution, palteontology and 

 embryology are thus brought into mutual bearing on one another. 



Let us take an example in which these two principles seem to be illustrated. 

 In rocks of the Silurian age there exist in great profusion the remarkable fossils 

 known as Graptolites. These consist of a series of little cups or cells arranged 

 along the sides a common tube ; and the whole fossil presents so close a resem- 

 blance to one of the Sertularian hydroids which inhabit the waters of our present 

 seas as to justify the suspicion that the Graptolites constitute an ancient and long 

 since extinct group of the Hydroida. It is not, however, with the proper cells, or 

 hydrothecEB, of the Sertulariaus that the cells of the Graptolite most closely agree, 

 but rather with the little receptacles which in certain Sertularinre belonging to the 

 family of the Plumularidte we find associated with the h3'drothec8e, and which are 

 known as " nematophores." A comparison of structure, then, shows that the Grap- 

 tolite may, with considerable probability, be regarded as representing a Plumu- 

 laria in which the hydrothecse had never been developed, and in which their place 

 had been taken by the nematophores. 



Now it can be shown that the nematophores of the living Plumularidse are filled 

 with masses of protoplasm which have the power of throwing out pseudopodia, or 

 long processes of their substance, and that they thus resemble the Rhizopoda, 

 whose soft parts consist entirely of a similar protoplasm, and which stand among 

 the Protozoa, or lowest gi-oup of the animal kingdom. If we suppose the hydro- 

 thecfe suppressed in a Plumularian, we should thus nearly convert it into a colony 

 of Rhizopoda, from which it would differ only in the somewhat higher morpho- 

 logical differentiation of its ccenosarc, or common living bond by which the indi- 

 viduals of the colony are organically connected. And just such a colony would, 

 under this view, a Graptolite be, waiting only for the development of hydrothecse 

 to raise it into the condition of a Plumularian. 



Bringing, now, the Evolution hypothesis to bear upon the question, it would 

 follow that the Graptolite may be viewed as an ancestral form of the Sertularian 

 hydroids, a form having the most intimate relations with the Rhizopoda, that 

 hydranths and hydrothecEe became developed in its descendants, and that the 

 Ilhizopodal Graptolite became thus converted in the lapse of ages into the hy- 

 droidal Sertularian. 



This hj'pothesis would be strengthened if we found it agreeing with the pheno- 

 mena of individual development. Now such Plumularidse as have been followed 

 in their development from the egg to the adult state do actually present well- 

 developed metamoi-phoses before they show a trace of hydrothecse, thus passing in 

 the course of their embryological development through the condition of a Grapto- 

 lite, and recapitidating within a few days stages which it took incalculable ages to 

 bring about in the palseontological development of the tribe. 



I have thus dwelt at some length on the doctrine of Evolution because it has 

 given a new direction to biological study, and must powerfully influence all futtire 

 researches. Evolution is the highest expression of the fundamental principles 

 established by Mr. Darwin, and depends on the two admitted faculties of living 

 beings — heredity, or the transmission of characters from the parent to the offspring, 

 and ■ adaptivity, or the capacity of having these characters more or less modified 

 in the offspring by external agencies or, it may be, by spontaneous tendency to 

 variation. 



The hypothesis of Evolution may not, it is true, be yet established on so sure a 

 basis as to command instantaneous acceptance ; and for a generalization of such 

 vast significance no one can be blamed in demanding a broad and indisputable 

 foimdation of facts. Whether, however, we do or do not accept it as a necessary 

 deduction from established facts, it is at all events certain that it embraces a greater 

 number of phenomena and suggests a more satisfactory explanation of them thau 

 any other hypothesis which has yet been proposed. 



