294 NaturaUHistorical Collections. 



disposed at their surface, and consequently of foliaceous appendages. The mul- 

 tiplication of their branches is purely accidental. Stems, on the contrary, are 

 always provided with vital knots on their surface, symmetrically arranged, or ac- 

 companied by a foliaceous appendage, — an organ sometimes reduced to a rudi- 

 mentary state, or altogether wanting. Potatoes, the bulbs of the Solanum tu- 

 berosum, are not roots, as generally supposed, but stems expanded at their ex- 

 tremities, and with the interior converted into feculent cells mixed with fibres. 

 And the same phenomenon is observed in the bulbs of the Jerusalem artichoke, 

 {Helianthus tuberosus.) But the Batatas, {^Convolvulus Batatas,) is a true tu- 

 berculous root — TuRPiN in Mem. du Mus. d'Hist. Nat. An. X. Cah. I. 



Marcel de Serres"" Zoological Periods Three principal causes, says M. Mar- 

 cel de Serres, * the lowering of the temperature, the retreat of seas, and inunda- 

 tions, have modified the surface of the globe, and destroyed a certain number of 

 beings, which in the beginning inhabited that surface. The most powerful of 

 these causes, the diminution of temperature, appears to have acted the first ; and 

 its effects have also been most extensive, since the continents were thereby solidi- 

 fied ; after which certain animals and plants must have ceased to exist as the 

 temperature of the earth decreased. The second, or the retreat of seas, has also 

 left numerous traces of its action. Regular in its effects, it has not, like inun- 

 dations, produced deposits out of the series, which do not exhibit that uniformity 

 and constancy found in deposits which were left by the seas as they retired from 

 the surface of our continents. 



The other causes which may have modified the crust of the globe, have been 

 very limited in tlieir effects, and have by no means exercised such a powerful in- 

 fluence on living beings as those of which we have spoken. 



When we observe the fossil remains of organized bodies, it is evident to us 

 that they have been deposited in the earth by successive generations, the most 

 simply organized being buried in the most ancient beds, and the most complicat- 

 ed in the most recent. We remark also that the remains of the same order, or 

 of the same formation, and more especially of the same stratum, have a parti- 

 cular resemblance to each other, and a general difference from those of supe- 

 rior or inferior deposits, or of other formations ; and this difference becomes 

 greater as the deposits are more distinct, and farther separated in a vertical direc- 

 tion. Thus, the organized bodies which have successively inhabited the earth, 

 are, with some exceptions, the more different from those now living, as their re- 

 mains are found inclosed in deeper strata, or as they have lived in times the more 

 remote from the present epoch. 



Organized beings having succeeded each other according to certain laws, the 

 most evident of which is their having appeared more slowly in proportion as their 

 organization was more complicated, are therefore as varied as the nature of 

 the strata which contain them ; whence, periods of animalization and of vegeta- 

 tion may be distinguished in the fossiliferous formations. 



In studying fossil animals in the order of their creation or of their distribution, 

 which indicates their successive formation, three great periods seem to be distin- 

 guishable. The first, or the most ancient, comprizes the space of time which 

 elapsed from the precipitation of the transition series, (or inferior secondary, ac- 

 cording to the language of M. de Serres,) to the deposition of the middle secon- • 

 dary formations. In this period, avertebral animals are singularly in excess over, 

 the vertebrated, which are reduced to a few traces of fishes ; the aquatic species- 

 far exceed in number the terrestrial species ; and some insects are the only animals, 

 with aerial respiration which have appeared at this epoch. The second period- 

 contains theentire series of secondary formations, (middle and superior;) it presents 

 a greater number of vertebrata, but principally of aquatic reptiles, with some ter,: 



• Geognosie des Terrains tertiaires, &c, 



