BRONN ON PAL.EONTOLOGICAL STATICS. 49 



milies at least, which were then in existence. In regard to forma- 

 tions as reciprocal palseontological equivalents, we think it necessary 

 to assume for our present purpose, perhaps, the following fifteen : 



a?, b, c, de, fg, ikl, m, n, op, q, r, f, t, uvwx 



f d O o cs H t-^ ^o p^ ^ o o n g 



I :§ 5 I I g- i" |g^ B'^ i g E g- I 



j^ j_- p 2 C3 • • cl 2 S Pj eg ^ 



P P 



-t 



x\lthough now, as we have seen, among 100 fossil species there may 

 be 12 passing from one formation into another, although further an- 

 other large part of the fossil species may have continued during the 

 whole time of such a formation, yet nevertheless a much more consi- 

 derable number, as we already indicated, have been limited merely to 

 |, |, ^, ^, ^ part of their corresponding formation, so that even within 

 one and the same formation a large part of the species or organisms 

 has been several times changed ; and it is assuredly not too high, if we 

 assume, that in each of the fifteen formations just adopted (a per- 

 haps excepted, for which however n furnishes more than a sufficient 

 compensation) the species have changed at least once, so that if some 

 lived throughout the whole period of a formation, others admitted 

 of a two- to threefold change. This change too has not taken place 

 synchronously for all, or even for the greater part of them, but gra- 

 dually, in like manner as the individuals of a species are born, some 

 early and others late, and perish, the one after a short, the other 

 after a long life ; but, nevertheless, the mean duration of a generation 

 may be determined, as, for instance, in the hum.an race, at thirty- 

 three years. Thus we may also assume the mean duration, or the 

 mean Mife of a species' (although we repeat it, only on the supposi- 

 tion that each species once existing completely filled up at least one 

 of these first formation-times have we found this mean duration 

 = 1 • 1 2 formation) = J the time of a formation when these are limited 

 to the above fifteen ; we would therefore, without recognising so 

 many universal synchronous renewals of the entire flora and fauna, 

 assume the whole time of the formation of the earth's crust as =30 

 changes of species, or thirty times the duration or ' life of a species.' 

 Turning now to the investigation of the other question, whether the 

 earth during each of these ' lives of a species' was as richly peopled 

 with species as at present, — at least in respect to those classes, or- 

 ders, and families which already existed, — we cannot hope to obtain 

 an answer by comparing the entire former flora or fauna with the 

 present, or by comparing the whole number of species of fossil rep- 

 tiles, fishes or mammalia with those now existing, but only when we 

 compare the fossil flora or fauna of some locality, peculiarly favour- 

 ably situated for the more perfect preservation of their remains, and 

 these collected, not out of a long series of strata, but at most from a 

 succession of strata corresponding to the Mife of a species,' with 



