BRONN ON PAL.EONTOLOGICAL STATICS. ;)•> 



few grouj3S of them may have again vanished, yet that those which 

 then existed were at all times almost as numerously represented hy 

 genera and species as at present ; although naturally in a systematic 

 point of view, greater or smaller oscillations hoth in a horizontal and 

 vertical direction were not thereby excluded, and many groups might 

 regularly be in reality somewhat less numerously, others also regu- 

 larly always more numerously represented than at present. The 

 objection, which some may make, that the species formerly were 

 more widely dispersed, and hence, though on the whole fewer might 

 yet be found as numerous, in one place, as we have seen above, can- 

 not have any essential influence on the result. 



It thus appears that we may base an estimate of the number of 

 species which have gradually peopled the surface of the earth on the 

 three following propositions : — (1) There has been at least thirty times 

 a change of species, or there have been thirty ' lives of a species ' on 

 the globe ; (2) in each of these ' lives of a species ' each group of the 

 vegetable and animal kingdom which then existed was represented 

 by as numerous species and genera as at present ; (3) notwithstand- 

 ing minute oscillations up and down of individual groups, the present 

 number of the species and genera of each group may be considered 

 as unity, as the equivalent of each * life of a species ' ; and these 

 oscillations may even be taken into account by means of an exponent 

 placed after the number of the existing species. We have assumed 

 it in the Cephalopods (but still too small) as = 1 00, in the entozoa 

 = ^, because formerly there were not so many classes of animals, 

 and consequently their entozoa must also be wanting. We have 

 finally taken that of the insects as only = ^, partly for similar reasons, 

 and partly in order that we might not, by too large an increase of 

 this number, which surpasses the sum of all the other species, possibly 

 prejudice too much the correctness of the whole. Thus we obtain 

 the following view of the duration and number of the various organic 

 beings during geological time : — 



VOL. V. — PART 11. H 



