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The position of Angiosperms as the highest plant-group is unassailable, but of the point or points of their origin from the general stem of the plant kingdom, and of the path or paths of their evolution, we can as yet say little. Until well on in the Mesozoic period geological history tells us nothing about Angiosperms, and then only by their vegetative organs.
There would be such cooler regions throughout the Jurassic, and we saw that there were considerable upheavals of land towards its close. To these elevated lands we may look for the development of the Angiosperms, the birds, and the mammals.
We next find that the Mesozoic is by no means purely an age of Gymnosperms. I do not mean merely that the Angiosperms appear in force before its close, and were probably evolved much earlier. The fact is that the Gymnosperms of the Mesozoic are often of a curiously mixed character, and well illustrate the transition to the Angiosperms, though they may not be their actual ancestors.
In fact, a careful study of the leaves preserved in the rocks seems to show the deciduous Angiosperms gaining on the evergreens at the end of the Cretaceous. The most natural, it not the only, interpretation of this is that the temperature is falling.
But it is impossible to retrace the lines of development of the innumerable types of Angiosperms. The geologist has mainly to rely on a few stray leaves that were swept into the lakes and preserved in the mud, and the evidence they afford is far too slender for the construction of genealogical trees.
In the feature of fruit and seed, by which the distribution of Angiosperms is effected, we have a distinctive character of the class.
The resemblance of the flora of Aix-la-Chapelle to the tertiary and living floras in the proportional number of dicotyledonous angiosperms as compared to the gymnogens, is a subject of no small theoretical interest, because we can now affirm that these Aix plants flourished before the rich reptilian fauna of the secondary rocks had ceased to exist.
Grasses also and palms begin in the Cretaceous; though the grasses would at first be coarse and isolated tufts. We will give some consideration later to the evolution of the Angiosperms. For the moment it is chiefly important to notice a feature of them to which the botanist pays less attention.
There is no doubt that the phylum of Angiosperms has not sprung from that of Gymnosperms. Within each class the flower-characters as the essential feature of Angiosperms supply the clue to phylogeny, but the uncertainty regarding the construction of the primitive angiospermous flower gives a fundamental point of divergence in attempts to construct progressive sequences of the families.
The trend of the evolution of the plant kingdom has been in the direction of the establishment of a vegetation of fixed habit and adapted to the vicissitudes of a life on land, and the Angiosperms are the highest expression of this evolution and constitute the dominant vegetation of the earth's surface at the present epoch.
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