Fig. 68.—Populus primæva, Heer. Cretaceous, of Greenland. One of the oldest known Angiosperms.
It is a remarkable fact in geological chronology that the culmination of the vegetable kingdom antedates that of the animal. The placental mammals, the highest group of the animal kingdom, are not known till the beginning of the Eocene Tertiary. The dicotyledonous Angiosperms, which correspond to them in the vegetable kingdom, occur far earlier—in the beginning of the Upper Cretaceous or close of the Lower Cretaceous. The reign of cycads and pines holds throughout the Lower Cretaceous, but at the close of that age there is a sudden incoming of the higher plants, and a proportionate decrease, more especially of the cycads.
I have already referred to the angiospermous wood supposed to be Devonian, but I fear to rest any conclusion on this isolated fact. Beyond this, the earliest indications of plants of this class have been found in the Lower Cretaceous. Many years ago Heer described and figured the leaves of a poplar (Populus primæva) from the supposed Lower Cretaceous of Komé, in Greenland ([Fig. 68]). Two species, a Sterculia and a Laurus or Salix, occur among fossils described by me in the upper part of the Kootanie series of the Rocky Mountains, and Fontaine has recently found in the Potomac group of Virginia—believed to be of Neocomian age—several angiospermous species (Sassafras, Menispermites, Sapindus, Aralia, Populus, &c.) mixed with a rich flora of cycads and pines. These are the early forerunners of the modern angiospermous flora; but so far as known they do not occur below the Cretaceous, and in its lower portions only very rarely. When, however, we ascend into the Upper Cretaceous, whether of Europe or America, there is a remarkable incoming of the higher plants, under generic forms similar to those now existing. This is, in truth, the advent of the modern flora of the temperate regions of the earth. A very interesting tabular view of its early distribution is given by Ward, in the “American Journal of Science” for 1884, of which the following is a synopsis, with slight emendations. I may add that the new discoveries made since 1884 would probably tend to increase the proportionate number of dicotyledons in the newer groups.
Dicotyledonous Trees in the Cretaceous.
| Upper Senonian (Fox Hill group of America.) | 179 | species. |
| Lower Senonian Upper white chalk of Europe; Fort Pierre group of America; coal-measures of Nanaimo? | 81 | species. |
| Turonian Lower white chalk; New Jersey marls; Belly R. group. | 20 | species. |
| Cenomanian. (Chalk-marl, greensand, and Gault, Niobrara and Dakota groups of America); Dunvegan group of Canada; Amboy clays of New Jersey. | 357 | species. |
| Neocomian (Lower greensand and Speeton clay, Wealden and Hastings sands, Kootanie and Queen Charlotte groups of Canada.) | 20 | species.[DK] |
[DK] Including an estimate of Fontaine’s undescribed species.
Thus we have a great and sudden inswarming of the higher plants of modern types at the close of the Lower Cretaceous. In relation to this, Saporta, one of the most enthusiastic of evolutionists, is struck by this phenomenon of the sudden appearance of so many forms, and some of them the most highly differentiated of dicotyledonous plants. The early stages of their evolution may, he thinks, have been obscure and as yet unobserved, or they may have taken place in some separate region, or mother country as yet undiscovered, or they may have been produced by a rapid and unusual multiplication of flower-haunting insects! Or it is even conceivable that the apparently sudden elevation of plants may have been due to causes still unknown. This last seems, indeed, the only certain inference in the case, since, as Saporta proceeds to say in conclusion: “Whatever hypothesis one may prefer, the fact of the rapid multiplication of dicotyledons, and of their simultaneous appearance in a great number of places in the northern hemisphere at the beginning of the Cenomanian epoch, cannot be disputed.”[DL]