1863.] DAVIDSON NOVA-SCOTIAST BRACM0P0DA. 161 



suppose, as many have done, that at the expiration of each of the 

 supposed great divisions of the sedimentary crust of the earth, 

 namely, the Palaeozoic, Mesozoic, and Cainozoic, there was a com- 

 plete extinction and renewal of life. Such an idea in 1863 would 

 be completely out of place ; for it has been shown over and over again 

 that many Palaeozoic genera have not only continued to be repre- 

 sented during a greater or less portion of the Mesozoic time, but 

 also throughout the entire geological sequence of sedimentary strata. 

 It is also quite evident that the admirable science of palaeontology, 

 notwithstanding its rapid strides and great discoveries, is far from 

 having revealed all its secrets, and is still in its youth. It cannot, 

 therefore, be expected that those who are now endeavouring to de- 

 cipher its difficult language can yet be in a position to furnish the 

 key to the genealogy of the numerous forms they daily meet with in 

 almost every rock and latitude, or that they can link together the 

 multitudinous and varied forms composing any class, and far less the 

 whole animal kingdom. 



It is quite true that, in the present state of our knowledge, there 

 appears to exist between the Palaeozoic and Mesozoic divisions or 

 periods a much wider break, or difference in the life-groups than 

 between the individual systems of which these divisions are com- 

 posed. It is, however, highly probable that, when our science is 

 more advanced, a closer agreement will be obtained. Experience has 

 taught us that no genus or species, once become extinct, is ever re- 

 created ; and it is to my mind quite certain that, as long as the same 

 genus is represented in any successive geological formations, this alone 

 is a positive proof that life was not interrupted during that period. 

 We are all aware that a certain number of genera of different classes 

 passed from the Palaeozoic into the Mesozoic period ; but I will not at 

 present enter into the discussion of this subject further than to men- 

 tion that, among the Bkachiopoda, Terebratula, Spirifera, Spiriferina, 

 Cyrt'ia, Athyris, Rhynclionella, Leptcena, Crania, Discina, and Lingula 

 passed from the Palaeozoic into the Mesozoic division. The Triassic 

 species have not yet been sufficiently compared with those from our 

 Palaeozoic deposits ; but a glance at some of the St. Cassian forms, or 

 even at Klipstein's and Minister's figures, will suffice to remove the 

 idea that there was a complete extinction of life at the close of the 

 Paloeozoic period and an entire renewal of species at the commence- 

 ment of the Mesozoic epoch. 



Geologists and palaeontologists are fully aware that the Permian 

 fauna is not nearly so rich in species as that of the Carboniferous period. 

 When we, therefore, compute the percentage of species common to 

 the two, the Permian fauna must be taken as the standard, not the 

 Carboniferous. The subject of the recurrence of Carboniferous species 

 in the Permian period is far from having been completely worked 

 out ; but, since 1845, it has attracted the attention of several com- 

 petent observers, amongst whom I may mention Messrs. de Yerneuil, 

 Brongniart, Goeppert, Gutbier, Geinitz, Kirkby, King, Howse, Eupert 

 Jones, and myself, in Europe ; Messrs. Meek and Hay den, Swallow 

 and Hawn, Dr. Shumard and Mr. Newbury, in America. 



vol. xix. — part i. si 



