1863.] DAVIDSON — NOVA-SCOTIAN BEACBIOPODA. 167 



been a complete discontinuance of sedimentary deposition over the 

 whole surface of the globe since the time when the first sediment 

 was formed, — that there has always existed a sea somewhere or 

 other upon the surface of the world, and that this sea has always 

 been inhabited. It is also certain that a number of favoured species 

 did escape for a time those local geological convulsions which have 

 taken place at numerous intervals; for the idea that universal 

 geological revolutions have ever taken place is now an all but ex- 

 ploded theory. Those hardy and favoured species have migrated 

 (as M. Barrande and others have so clearly shown), and continued to 

 exist, if not in their original home, in some other more distant place, 

 for a greater or less lapse of time, while the larger number of the 

 others may have been suddenly or gradually destroyed by those great 

 changes in the configuration of land and sea which have no doubt 

 taken place gradually or periodically. 



It is not, therefore, surprising to find certain forms of an older 

 period in newer strata, or to find certain species striving to exist for 

 a greater or less extent of geological time, irrespective of the geolo- 

 gical system in the strata of which they are imbedded. 



Time and assiduous research will show how far we may speculate 

 upon the origin of species ; but, in the present state of palseontolo- 

 gical science, all theories built upon such subjects, however ad- 

 mirably handled, as is that by Darwin, must, before admission, be 

 tested by more unmistakeable evidence than science is at present in 

 a condition to offer. 



"We cannot tell, in the present state of our knowledge, whether the 

 species of any class, be it Echinodermata,BracMopoda, Crustacea, Pisces, 

 or any other, have been derived from a single or a hundred original 

 progenitors. I will not say that future researches may not simplify 

 the question, but in the present state of our experience we are very far 

 from having arrived at any such conclusion. A similarity of original 

 type has sometimes, by exception, occurred during a very prolonged 

 period : thus, for instance, we have certain forms of Lingula, Crania, 

 Discina, Rhynchonella, &c, which, to all outward appearance, appear 

 to have varied but little during all the sequence of geological time ; 

 there is one form of Crania, for example, which occurs in the Silu- 

 rian, Devonian, Carboniferous, Permian, Jurassic, Cretaceous, and 

 Tertiary strata, and in the recent state, which, although possessing 

 as many distinct denominations, is hardly specifically distinguishable, 

 while other species of that genus varied considerably in the different 

 geological periods. 



But to return to the more immediate subject of this communica- 

 tion, I may observe that, although I quite agree with M. Marcou 

 that there exist in the Permian formation some few forms which 

 may recall to mind some which existed in the Mesozoic period, it 

 must be allowed that that number forms the minority, and that, on 

 the contrary, the great bulk of the Permian species, to whatever class 



perhaps more in our idea than in nature, and conforming more to the actual 

 state of science than to its complete development. (Bull. Soc. Geol. France, 

 2 me ser. vol. xix. p. 612, 1862.) 



