iiS GEOLOGICAL MEMOIRS. 



rock-member of one period to the first member of the next period ; 

 nevertheless, this depends not only on the accidental character of the 

 two kinds of rock, bnt also rests on the supposition of a uniform 

 proportion between the time and the causes that destroy species. 

 Would we lastly, without concerning ourselves about the time, 

 equalize the periods with each other merely in this manner, that they 

 should only contain an equally small number or quota of common 

 species, it may be again asked, whether the continuous preservation 

 and the relative extinction of old species forms an element better 

 adapted for a measure, than the appearance of new species ? For 

 thus the cretaceous (IV.) has a larger quota of species in common 

 with the existing period (VI,), than any two former periods imme- 

 diately bordering on each other have in common, and yet no other 

 periods are so distinctly separated from each other as the chalk is 

 from the tertiary, by the above-mentioned appearance of the highest 

 forms of plants and animals, in the former of a part of the Mono- 

 chlamydeae, the Corolliflorse and Choristopetalee, in the latter of a 

 part of the fishes (osseous fishes), of the reptiles (serpents and ba- 

 trachians), and of the two classes of the warm-blooded vertebrata. 



Were we to arrange the periods according to their absolute rich- 

 ness in fossil species they v/ould stand thus : — 



According to the Plants . . IV, II, III, V, I. 



„ Animals . II, III, I, IV, V. 



both together, II, III, IV, I, V. 



The carboniferous period, from the accumulation of carbonaceous 

 and clayey materials in its rocks, was the most favourable for the 

 preservation of plants, and hence it furnishes us with quite as many 

 species of them as all the other periods together, though a portion 

 of the system, now three times the most numerous, first appears in 

 the last of them. The Trias period (II.) is evidently, not merely from 

 accident, but in reality poorer, and undoubtedly shorter and of a more 

 local character than the others. The Cretaceous period (IV.) con- 

 tains almost no rocks adapted for the preservation of plants, and in 

 especial entirely wants land and freshwater formations. The Oolite 

 period (III.) may in this respect be design«ated as the true, indifferent 

 mean (or centre) of the periods. The Tertiary period (V.) finally is 

 essentially distinguished by a greater richness in organic species in 

 general, and in animals in particular, which appears to be a conse- 

 quence not merely of a greater capacity in the rocks for preserving 

 them, nor of a probably longer continuance of the period, but of an 

 essentially greater richness of the time in all grades of organic forms. 

 We reserve some other questions for illustration on another occa- 

 sion. J. N. 



