46 GEOLOGICAL MEMOIRS. 



out any possibility of finding any determinate limit between tbem. 

 R. Owen has recognized in the English (newer-pliocene) tertiary strata 

 in 40 species of mammalia, 30 ( = 0' 75) as still li\ing. In this respect, 

 however, there appears an essential distinction, since only the tertiary 

 passes thus gradually into the existing period, whereas between all the 

 earlier periods some limit has hitherto been found where the number 

 of transition species is very small, and forms only an inconsiderable 

 proportion (=0"01 — 0*03), a circumstance which might readily lead 

 to the view, that a community of species, at least between different 

 periods, was to be wholly denied. — These species have both begun to 

 exist at different times within the periods, and have existed for dif- 

 ferent times ; epochs of the synchronous origin and synchronous 

 perishing of an entire creation have never once occurred. — The length 

 of the duration of a species is very unequal ; it can, as we have seen, 

 extend into three or two periods, continue for 8 — 5 — 3 — 2 — 1 for- 

 mations, nay even only for a portion of a formation, for one or two 

 of its subordinate beds. This leads us to inquire into the mean du- 

 ration of a species. According to the numbers in the Enumerator, 

 with the exclusion of species still living, there pass 



of 2055 plants 1 2 or 0-006 1 



of 24,366 animals . . . .3322 or 0*134 ^ species into other formations, 

 of 26,421 organisms . . 3334 or 0-124 J 



a proportion that for the plants is too small, as in it the carboni- 

 ferous plants in the lias of the Tarentaise have not been twice enu- 

 merated, whilst in general it must be considered, that many species 

 only appear in two or more formations in consequence of erroneous 

 determinations, — that these cases are chiefly confined to the Amor- 

 phozoa, Phytozoa, Anthozoa, and Conchylise, with the Trilobites 

 («-f 6), since the other classes have been more thoroughly elaborated, 

 and especially among the mammalia the occurrence in two forma- 

 tions, chronologically distinct, almost never happens ; that, if the 

 heads v and x, in agreement with a former observation, were cancelled 

 as synchronous with, other formations, the most numerous instances 

 of transitions would disappear ; whereas some species overleap one 

 or more formations, and hence very probably, at least in general, 

 must also be introduced into the intermediate members : relations, 

 with due regard to which the average duration of the species for in- 

 dividual groups may be calculated. From this it may, on the whole, 

 be deduced that each species has had an average duration of less than 

 1*12 formation, to which must also be added the very essential con- 

 sideration, that the occurrence in one period is not an occurrence 

 during this whole period, but rather, according to observations, for the 

 details of which space is wanting in our work, must be assumed as 

 much shorter on the average. Murchison and De Verneuil have also 

 established the vieAv, to which we had long attained, that those 

 species, which possess the greatest geological duration, are also those 

 which have a wide geographical distribution. 



