48 GEOLOGICAL MEMOIRS. 



the genera may have the same duration (Monomya), or it may he 

 composed of more Umited and more extended subgroups (Brachio- 

 pods of the Genuina and Rudistse), or again consist entirely of 

 limited subdivisions with limited genera (Pteropods). 



III. Number of the Species. 



"We have enumerated 2055 species of plants, 24,366 of animals, 

 together 26,421 fossil species, which, as already mentioned, after 

 excluding the synonymous species, may perhaps be reduced about 

 O'lO, in some classes even 0'20. The fossil plants make also 0*08 

 of the fossil animals ; and the proportion of the fossil species to the 

 living is, in romid numbers, in the 



Fossil. Living. Fossil & living. 

 Plants = 2050: 70,000: 72,050= 3:100:103 

 Animals = 24,000 : 100,000 : 124,000 = 24 ; 100 ; 124 



Both =26,250 : 170,000 : 196,050= 15 : 100 : 115 



Whilst therefore the number of species of living animals is not much 

 greater than that of li^-ing plants (100 : 70), that of fossil animals 

 surpasses the fossil plants in a much higher ratio (=100 : 9). But 

 assuredly a proportion between plants and animals so widely differ- 

 ent from the present has never formerly existed, since the two king- 

 doms generally, — in individual families, genera and even species, — 

 exercise so great a reciprocal influence on each other, that a great 

 increase or multiplication of the one kingdom is not possible without 

 that of the other. It is quite certain that formerly, not only relatively 

 many more plants, but also many more insects, birds, soft mollusca, 

 &c., even more land reptiles and mammalia have existed, compared 

 with the conchyliBe, than the strata now show us, since these are 

 not all so well adapted for the reception and preservation of every 

 class of beings as for that of the conchy lise. We have therefore 

 proposed the question to ourselves, whether, — presupposing the pre- 

 sent proportion of the separate divisions of the organic kingdoms to 

 each other to have prev ailed so long as these divisions can be proved 

 to have existed — it is not possible from the number of still living- 

 species to estimate that of all that have existed, whilst we calculate 

 from the number of preserved species in the easily preserved classes 

 of animals, the number of species that once existed in the difficultly 

 preservable classes, orders, &c. of animals and plants, from the num- 

 ber of parasites the number of the species on which they lived and 

 the reverse, on the supposition of a similar numerical proportion to 

 the present, from the time of the certain appearance of each of these 

 groups to that of its disappearance from the earth's surface, or to the 

 commencement of the present period ? For this purpose we must 

 first come to a more precise decision regarding the number of exist- 

 ing formations in a palaeontological point of view, and then endeavour 

 to furnish the proof, that the earth was actually at each time as fully 

 and variously peopled as at present by those classes, orders and fa- 



