Notices of Memoirs — De Saporta — Jurassic Plants. 275 



of geologists, who have liberally given me access to their collections, 

 as well as the patronage and advice of M. Ad. Brongniart. 



By a coincidence which it is natural to mention here, the Histoire 

 des Vegetaux Fossiles — the eminent but incomplete work of the 

 illustrious French savant — stops just towards the end of the Crypto- 

 gams, and thus excludes the greater number of the Jurassic species, 

 some because they belong to the class of Gymnosperms, others 

 because they were not known at the time when M. Brongniart pub- 

 lished bis Histoire. This work is thus taken up almost at the point 

 where M.. Brongniart's has left off. The need of the publication is 

 obvious from this statement ; the subject is in itself interesting. The 

 Jurassic period, from a biological point of view, constitutes a kind 

 of middle age, equally distant from the Pala30zoic and Neozoic 

 periods. It serves, so to speak, as a hyphen between epochs which, 

 without it, would present a complete contrast ; but this hyphen 

 itself corresponds to a very long period. 



The configuration of the European land modified several times, 

 the deposits varying in their nature and aspect, new series of marine 

 animals substituting themselves for former ones, and eliminated in 

 their turn by others, — ^all these phenomena, by their intensity and 

 repetition, bear witness to the immensity of the period. Neverthe- 

 less, it is to be observed, that the vegetation appears to have changed 

 less than anything else. Not only has it preserved longer than the 

 population of the sea the species it contained at a given time in the 

 period, but its general characters and the relative disposition of its 

 elements have sufiered far less alteration from the lapse of time : in 

 a word, it has remained almost stationary from one end of the period 

 to the other, instead of visibly progressing, as is proved, respecting 

 the Cretaceous plants, by comparing those which existed in the 

 Wealden with the Flora of the white Chalk or Santonien. That is, 

 in my opinion, the principal feature of the Jurassic- vegetation. 

 Consult from this point of view the Keuper, the Ehastic, the Oolite, 

 or the Wealden ; that is to say, place yourself in the age which 

 immediately precedes the period, at its beginning, middle, or even at 

 the end, there is almost always the same general physiognomy to be 

 observed; and the Ferns, the Eqtiisetacece, the Cycadacece, and the 

 Coniferce, that one meets, are combined in relative proportions which 

 vary very little. A second phenomenon, which is not without con- 

 nexion with the last, consists in the reourrence of similar, but not 

 absolutely identical forms, although it is difficult at times to dis- 

 tinguish them, which have just shown themselves in successive 

 stages separated by intervals moi'e or less long, as if the same forms 

 re-appeared still recognizable, although slightly modified. 



It is thus that several Rhaetic species seem to re-appear in the 

 Oolite, and some of these, like the Baiera digitata, Schimp., to return 

 in the Wealden under the name of Baiera plnripartita, Schimp. 

 One cannot reasonably assign to these singular parallelisms, which I 

 have attempted to sum up, any other cause than the persistence or 

 the reproduction of the same physical conditions, bringing with it 

 the preservation or the return of the same organic combinations. 



