^*«") LABORS OF BRONGNIART. 407 



lowest to the highest forms of vegetation. He believed in the gradual 

 reduction of temperature in the climate of the globe from the earliest 

 times, and in the purification of the atmosphere from a former excess of 

 carbonic acid, favorable only to the lower types which then prevailed. 

 He divided the geologic series into four great periods, the first extending 

 through the Carboniferous, the second embracing the gres bigarr6, or 

 Bnntersandstein, only, the third seeming to include the rest of the Trias, 

 the Jurassic, and the Cretaceous, and the fourth completing the series. 

 The table which he gives on page ^ is calculated to show the develop- /© 

 ment of the higher types of vegetation in successively higher strata, and 

 may profitably be compared with the one having the same form, which 

 will be found below {infra, pp. 440-441). Of this table he remarks that 

 in the first period there exist hardly anything but Cryptogams, plants 

 having a more simple structure than that of the following classes. I n the 

 second period the number of the two following classes becomes propor- 

 tionately greater. During the third period it is the Gymnosperms which 

 specially predominate. This class of plants may be considered interme- 

 diate between the Cryptogams and the true Phenogams (Dicotyledons), 

 which preponderate during the fourth period. The words italicized 

 in the liberal translation here made are scarcely less than a prophecy, 

 and one whose fnlflllment is only now being tardily granted by system- 

 atic botanists. In this tabular exhibit Brongniart enumerates 501 spe- 

 cies of fossil plants known to him, 240 of which belonged to the first period 

 (Paleozoic), 25 to the second,. 72 to the third, and 164 to the fourth. He 

 also states the number of living species at 50,350. A comparison of 

 these figures with those of our own time, as given in the table below, 

 will afford a sort ot measure by which to judge of the nineteenth cen- 

 tury as an era of scientific discovery. 



Brongniart propounded a theory for the primordial distribution of 

 land vegetation over the globe which is well worth a passing notice, and 

 is not weakened by modern theories of post-glacial distribution, which 

 might also be true. His theory, in brief, was that it began on small 

 islands, the only land then existing ; that these islands became gradu- 

 ally united and consolidated into continents upon which a different veg- 

 etation, more varied, and more like the present vegetation could exist, 

 and he says that it was not until after the formation of the chalk {i. e., the 

 beginning of the Tertiary) that such a continental vegetation seemed 

 to have appeared. He concludes from this that it was from this period 

 that large areas of the earth's surface began to be laid bare, and that 

 true continents commenced to be formed. He regarded it as remarka- 

 ble that great changes in both the flora and the fauna of the globe 

 should have taken place almost simultaneously ; that the age of Cycads 

 should correspond with that of reptiles and the age of Dicotyledons 

 with that of mammals (p. 221). But unless fresh discoveries of this 

 last-named class of animals shall be hereafter made in the middle Cre- 

 taceous we must regard this second coincidence as now disproved. 



