WARD.] DISCUSSION OP DIAGRAMS. 449 



Not only must a new form of growtli from the seed and from the ground 

 be developed, and a sort of bilateral symmetry be introduced, but in ad-" 

 dition to this, and, as I believe, in great part due to it, the exogenous 

 mode of circulation and tissue growth must supplant the endogenous 

 one, whereby the stem may increase in thickness as well as in length. 

 These great mechanical problems were worked out during Mesozoic 

 time and in the Middle Cretaceous, represented in this country by the 

 Dakota group, and in Europe by the Cenomanian epoch, the great type 

 of plant life appeared which was destined to dominate the world and 

 sink all other forms into insignificance. But the most astonishing fact 

 is that this young giant was born, as it were, full grown. In this lowest 

 horizon at which anj' Dicotyledons appear^^''' we have already obtained 

 more than three hundred species belonging to all three of the great 

 divisions of the subclass, and exhibiting ample, luxuriant foliage. They 

 embrace many of our most familiar forms, the poplar, the birch, the 

 beech, the sj'camore, and the oak. Here appears the tig tree, the true 

 laurel, the sassafras, the persimmon, the maple, the walnut, the mag- 

 nolia, and even the api)le and the plum. We must conclude, then, that 

 the Dicotyledonshad a much earlier origin than is shown by our defective 

 record, and that they had been long developing through the Mesozoic 

 ages. 



If now we follow the advancing wave of plant life from this point up- 

 ward we shall see that from ^he new vantage-ground furnished by the 

 closed ovary, the perfect flower, and the exogenous trunk, its march 

 was rapid and steady until we reach the Miocene Tertiary, the culmi- 

 nating point in the paleontological series. Here the species actually 

 found are numbered by thousands, and the higher types greatly pre- 

 dominate over the lower ones. But from this point the record begins 

 to fail, and can no longer be trusted. Very little is found in the Plio- 

 cene, and still less in the Quaternary; but this cannot indicate an ac- 

 tual decline in these types of vegetation. It must be due to the ap- 

 proach of a state of things which rendered the preservation of vegetable 

 remains diflacult, a condition, as already remarked, which is especially 

 characteristic of the present state of the globe. The march of the Di- 

 cotyledons was uninterrupted, and still continues. The figures given in 

 the numerical table represent, in round numbers, the (stiinates of 

 Messrs. Bentham and Hooker, as given in their " Genera Tlantarum," 

 and may, therefore, be taken as the most reliable that can be obtained. 

 The three divisions of the Dicotyledons combined amount to 87,000 spe- 

 cies, and constitute nearly 60 per cent, of the flora of the globe, 



With regard to the three divisions of the Dicotyledons, although they 

 are all represented in the lowest formation at which any considerable 

 number are found, still the Apetalse constitute a larger proportion of the 

 Dicotyledons in the Cenomanian (45 per cent) than in the Miocene (37 



2" If we accept the solitary Fopulus jarimwva, Heer, from the Urgonian beds of 

 Kome, Greenland. 



GEOL 84 29 



