That all VASCULAR plants really belong to one stock seems certain, and here the palaeontological record has materially strengthened the case for a monophyletic history. The Bryophyta are not likely to be absolutely distinct, for their sexual organs, and the stomata of the Mosses strongly suggest community of descent with the higher plants; if this be so it no doubt establishes a certain presumption in favour of a common origin for plants generally, for the gap between "Mosses and Ferns" has been regarded as the widest in the Vegetable Kingdom. The direct evidence of consanguinity is however much weaker when we come to the Algae, and it is conceivable (even if improbable) that the higher plants may have had a distinct ancestry (now wholly lost) from the beginning. The question had been raised in Darwin's time, and he referred to it in these words: "No doubt it is possible, as Mr G.H. Lewes has urged, that at the first commencement of life many different forms were evolved; but if so, we may conclude that only a very few have left modified descendants." ("Origin of Species", page 425.) This question, though it deserves attention, does not immediately affect the subject of the palaeontological record of plants, for there can be no reasonable doubt as to the interrelationship of those groups on which the record at present throws light.
The past history of plants by no means shows a regular progression from the ****** to the complex, but often the contrary. This apparent anomaly is due to two causes.
1. The palaeobotanical record is essentially the story of the successive ascendancy of a series of dominant families, each of which attained its maximum, in organisation as well as in extent, and then sank into comparative obscurity, giving place to other families, which under new conditions were better able to take a leading place. As each family ran its downward course, either its members underwent an actual reduction in structure as they became relegated to herbaceous or perhaps aquatic life (this may have happened with the Horsetails and with Isoetes if derived from Lepidodendreae), or the higher branches of the family were crowded out altogether and only the "poor relations" were able to maintain their position by evading the competition of the ascendant races; this is also illustrated by the history of the Lycopod phylum. In either case there would result a lowering of the type of organisation within the group.
2. The course of real progress is often from the complex to the ******.
If, as we shall find some grounds for believing, the Angiosperms came from a type with a flower resembling in its complexity that of Mesozoic "Cycads," almost the whole evolution of the flower in the highest plants has been a process of reduction. The stamen, in particular, has undoubtedly become extremely simplified during evolution; in the most primitive known seed-plants it was a highly compound leaf or pinna; its reduction has gone on in the Conifers and modern Cycads, as well as in the Angiosperms, though in different ways and to a varying extent.
The seed offers another striking example; the Palaeozoic seeds (if we leave the seed-like organs of certain Lycopods out of consideration) were always, so far as we know, highly complex structures, with an elaborate vascular system, a pollen-chamber, and often a much-differentiated testa. In the present day such seeds exist only in a few Gymnosperms which retain their ancient characters--in all the higher Spermophytes the structure is very much simplified, and this holds good even in the Coniferae, where there is no countervailing complication of ovary and stigma.
Reduction, in fact, is not always, or even generally, the same thing as degeneration. Simplification of parts is one of the most usual means of advance for the organism as a whole. A large proportion of the higher plants are microphyllous in comparison with the highly megaphyllous fern-like forms from which they appear to have been derived.
Darwin treated the general question of advance in organisation with much caution, saying: "The geological record...does not extend far enough back, to show with unmistakeable clearness that within the known history of the world organisation has largely advanced." ("Origin of Species", page 308.)Further on (Ibid. page 309.) he gives two standards by which advance may be measured: "We ought not solely to compare the highest members of a class at any two periods...but we ought to compare all the members, high and low, at the two periods." Judged by either standard the Horsetails and Club Mosses of the Carboniferous were higher than those of our own day, and the same is true of the Mesozoic Cycads. There is a general advance in the succession of classes, but not within each class.
Darwin's argument that "the inhabitants of the world at each successive period in its history have beaten their predecessors in the race for life, and are, in so far, higher in the scale" ("Origin of Species", page 315.)is unanswerable, but we must remember that "higher in the scale" only means "better adapted to the existing conditions." Darwin points out (Ibid. page 279.) that species have remained unchanged for long periods, probably longer than the periods of modification, and only underwent change when the conditions of their life were altered. Higher organisation, judged by the test of success, is thus purely relative to the changing conditions, a fact of which we have a striking illustration in the sudden incoming of the Angiosperms with all their wonderful floral adaptations to fertilisation by the higher families of Insects.
II. PHYLOGENY.
The question of phylogeny is really inseparable from that of the truth of the doctrine of evolution, for we cannot have historical evidence that evolution has actually taken place without at the same time having evidence of the course it has followed.