BRONN ON PALiEONTOLOGlCAL STATICS. 47 



II. Duration of the Genera. 



There are natural genera {Sippen), which, even although they con- 

 tain several species, are limited to a single formation, whilst others 

 pass through several formations, several periods, all periods, and even 

 enter into the existing creation. Thus we find numbered 



in different 



Periods. Formations. Period. Form. 



Plants the 350 genera 463, 592 times = 1 : 1-32 : 1-69 



Animals 2501 „ 3347, 5415 times = 1 : 1-34 : 2-17 



Together 2851 „ 3810, 6007 times = 1 : 1-34 : 2-11 



Among 100 genera therefore 34 pass into another period, and 100 

 genera of plants occur 169 times, 100 of animals 217 times, and 100 

 of both together 211 times in different formations (hence 69, 117, 

 111 times in a second or other formation). This proportion will, 

 however, become smaller by the elision of the formations "Ik, v and 

 Xy and be increased when we take into account that many genera oc- 

 cur in two formations or periods between which they are wanting 

 in 1 — 2 others, but yet probably have existed and hence must be 

 reckoned or supposed to exist, — excepting however those cases where 

 genera are unnaturally composed of heterogeneous species, so that 

 the older species cannot remain united in one genus with the more 

 recent. It is usual to suppose (with Forbes) each genus, during its 

 geological continuance, increasing in species to a point in time of 

 greatest development, and from that again decreasing to its gradual 

 extinction, where indeed this point in time of maximum development 

 does not fall in the earliest silurian or the existing period. Never- 

 theless though this form of development occurs in some large genera 

 (very small genera furnish no measure or have no form), it is not 

 the usual one ; we rather find that in general, between the pretty 

 rapid or occasionally sudden increase or decrease of species, their 

 number in the separate formations or periods remains rather constant. 

 The lower families of plants and animals — which on the whole in 

 this and other conditions of their occurrence bear a closer relation 

 to each other, than the lower to the higher families of plants or 

 the lower to the higher families of animals — contain tiie genera 

 of longest duration ; thus whilst several genera of marine algae 

 among the cellular plants, and the marine polyps, annelids and 

 especially mollusca among invertebrate animals, continue through the 

 whole series of formations and even into the present creation, the 

 genera of vascular plants, of the other entomozoa, and the whole ver- 

 tebrata are limited to shorter periods, so that almost all continue only 

 for a few periods, or mostly for one period, and those of the birds 

 and mammalia at the most belong only to one period and almost 

 always to one formation, so far as they do not pass into the present 

 creation. In a geologically limited class or order of organisms all 

 the genera must also necessarily be so (Mammalia, Choristopetalse) ; 

 in a geologically extensive one on the contrary, either (almost) all 



