44 GEOLOGICAL MEMOIRS. 



genera of organisms, whose remains, occurring too only as rarities, 

 are so imperfect, so crushed, so small, so unlike those parts on which 

 existing genera depend, as is the case in regard to insects ? How 

 shall we distinguish genera from such unimportant external parts as 

 the shells of the Asiphonobranchise among the molluscs, a group, of 

 which the soft inhabitants even in the existing world are for the most 

 part not examined, but only distributed by chance among the esta- 

 blished genera of shells ? How shall we recognise fossil genera from 

 the leaves and wood of plants, when we are not able to determine even 

 existing genera from their leaves and wood ? 



It thus indeed seems too early to institute a comparison between the 

 present and former creations ; these difficulties must first be set aside, 

 these doubts solved, these deficiencies filled up ! But will they ever all 

 be so ? Will even a considerable portion of them soon be so ? We are 

 not bound to wait for this, but only, when instituting this compari- 

 son, to keep in remembrance, that all the imperfections just mentioned 

 attach to this comparison. It is necessary for us to remember that 

 when we express the results of this comparison in mathematically 

 precise terms, these yet are only inexact, approximate values, accord- 

 ing to the present momentary condition of our knowledge, — that those 

 results which are deduced only from single small numbers, are of 

 much inferior value to those which depend on the combination of 

 higher numbers, — that the image {Bild) here given depends on the 

 sum of the previous considerations, and that new observations may in 

 the course of years very considerably modify it, even although many 

 of the results it contains must already be regarded as firmly esta- 

 blished for ever. 



[In explanation of the following tables we must premise that the columns with 

 the headings a g, m p, g f and s x, contain those sums arising from the addition of 

 the columns between these two letters (ag = abcdefg), which sums naturally are 

 too large, since many species, genera, &c. occur more than once in them, and on 

 this account immediately after, under I, II, III, IV, V, the true sum of the genera 

 and species is given. This also happens in the last column but one of Table II. 

 with the lines a — x and I — V, of which the latter is again too large, and the true 

 sum is therefore given in the last column*.] 



* As stated above, Professor Bronn, in preparing his summary, has occasionally 

 made allowance for errors in the lists, and the number of species in these Tables 

 consequently varies slightly from that given in the Journal, p. 33, above, which 

 was drawn up from the work itself. The Plantce cellulares are there also erro- 

 neously stated at 773 instead of 193, the number of names in the list. — J. N. 



