Tree fern.
To understand the problem from the Earth’s point of view, let us review the facts with which geology presents us. The flora of paleologic times, as we see both at their advent in the Devonian and from their superb development in the Carboniferous era, consisted wholly of forms whose descendants now seek the shade.[18] Tree ferns, sigillaria, equisetæ, and other gloom-seeking plants composed it. That some tree fern survivals to-day can bear the light does not invalidate the racial tendency. We have plenty of instances in nature of such adaptability to changed conditions. In fact, the dying out and deterioration of most of the order shows that the conditions have changed. And these plants, grown to the dimensions of trees, inhabited equally the tropic, the temperate, and the frigid zones as we know them now. Lastly, no annual rings of growth are to be found on them.[19] In other words, they grew right on, day in, day out. The climate, then, was as continuous as it was widespread.
On the other hand, astronomy and geology both assert that the seas were warm.[20] From this it follows that a vastly greater evaporation must have gone on then than now, and that a welkin of cloud must thus inevitably have been formed.
Now put the two facts together, and you have the solution. The climate was warm and equable over the whole globe because a thick cloud envelope shut off the Sun’s heat, the heat being wholly supplied from the steamy seas. At the same time, by the same means the light was necessarily so tempered as to produce exactly that half-light the ferns so dearly love. One and the same cause thus answers the double riddle of greater warmth and less light in those old days than is now the case.
And here comes in the second find I spoke of above, in the person of some old trilobites who stepped in unexpectedly in corroboration. It has long been known—though its full significance seems to have escaped notice—that in 1872 M. Barrande made the discovery that many species of trilobites of the Cambrian and lower Silurian, the two lowest, and therefore the oldest, strata of paleozoic times, and distant relative of our horseshoe crabs, were blind. What is yet more significant, the most antediluvian were the least provided with eyes. Thus in the primordial strata, one-fourth of the whole number of species were eyeless, in the next above one-fifth, and in the latest of all one two-hundredth only.[21] Furthermore, they testify to the difficulty of seeing, in two distinct ways, some by having no eyes and some colossal ones, strenuous individuals increasing their equipment and the lazy letting it lapse. It seems more than questionable to attribute this blindness to a deep-sea habitat, as Suess does in describing them, for they lived in what geologists agree were shallow seas on the site of Bohemia to-day. Besides, trilobites never had abyssal proclivities; for they are found preserved in littoral deposits, not in deep-sea silt. Muddy water may have had some hand in this, but muddy water itself testifies to great commotion above and torrential rains. So the light in those seas was not what it became later, or would be now. Thus these trilobites were antelucan members of their brotherhood, and this accuses a lack of light in those earlier eras even greater than in Carboniferous times, which is just where it ought to be found if the theory is true.
I trust this conception may prove acceptable to geologists, for it seems imperative from the astronomic side that something of the sort must have occurred. And it is just as well, if not better, to view it thus in the light of the dawn of geologic history as to remain in the dark about it altogether. Nescience is not science—whether hyphenized or apart; for the whole object of science is to synthesize and explain. Its body of learning is but the letter, coördination the spirit, of its law. Nevertheless, the unpardonable impropriety of a new idea, I am aware, is as reprehensible as the atrocious crime of being a young man. Yet the world could not get on without both. Time is a sure reformer and will render the most hardened case of youth senile in the end. So even a new idea may grow respectable at last. And it is really as well to make its acquaintance while it still has vigor in it as to wait till it is old and may be embraced with impunity. Boasted conservatism is troglodytic, and usually proves a self-conferred euphuism for dull. For conservatism proceeds from slowness of apprehension. It may be necessary for certain minds to be in the rear of the procession, but it is of doubtful glory to find distinction in the fact.
Thus the youth of a world, like the babyhood of an individual, is passed screened from immediate contact from without. That this is the only way that life can originate on a planet we cannot say, but that it is away in which it does occur, our own Earth attests, and that, moreover, it is the way with all planets of sufficient size, the present aspect of the major planets shows. It may well be that with celestial bodies as with earthly species, some swaddle their young, others cast them forth to take their chance, and that those that most protect them rear the higher progeny in the end. What glories in evolution thus await the giant planets when they shall have sufficiently cooled down, we can only dimly imagine. But we can foresee enough to realize that we are not the sum of our solar system’s possibilities, and by studying the skies read there a future more wonderful than anything we know.