The question after all, says Professor Gray, is not "how plants and animals originated, but how they came to exist where they are, and what they are." On only one of these points--that of favoring conditions--can any satisfactory answer be given, except as we defer to the Bible genesis, which explains all. And the reason is, that we can never determine what forms are specific without tracing them back to their origin, and this is impossible. Orders, genera, species, etc., are only so many lines of thought on which we arrange our classifications, just as the parallel wires of an abacus, with their sliding balls, are the lines on which we make our mathematical computations. Agassiz would not allow that varieties existed in nature, except as man's agency effected them, that is, as they were brought about by artificial processes.
These artificial processes are quite numerous, and many of them have been practised from remote antiquity. But they seem to have no counterpart in nature, except as insects may contribute to modifications by the distribution of pollen. But all modifications of this character tend towards infertility, while few plants accept any fertilizing aid from other and different species. Any break in their hereditary tendencies, resulting in a metamorphosis that involves the integrity of their stamens and pistils, is stoutly resisted by nature. In considering the question of species, therefore, we should confine our observations to those produced by natural, not artificial, methods; to plants as propagated by the loving tendencies of nature, not by the arbitrary and exacting methods of man--those looking to his gratification only. All these fall into the category, of "nature's bastards," as Shakespeare happily defines them. In view of these considerations, and the new methods of classification, such as grouping genera into families or orders, and these into sub-orders, tribes, sub-tribes, etc., we can readily understand why the great Harvard Professor should have wholly eliminated community of descent from his idea of "species," or hesitated to regard varieties otherwise than as the result of man's agency.
Indeed, the whole question of species, as well as varieties, is likely to undergo material modifications in the future. On some points the botanists and zoologists differ widely already, many making likeness among individuals a secondary consideration, and genealogical succession the absolute test of species. Others, on the contrary, make resemblance the fundamental rule, and look upon habitual fecundity within hereditary limits as provisional, or answering to temporary needs only. These differences of opinion would seem to be the more tenaciously held as the question of new varieties presses for solution at the hands of nature, rather than by the agency of man. All these varieties tend less to new races than to cluster about type-centres, and can go no further than certain fixed limits of variation, beyond which all oscillations cease. But none of these questions touch the real marrow of the controversy as to origin, or aid us in determining the duration of species.
The presence of the two great families of trees--the sequoias and the oaks--as far back as the Miocene period, if not extending through the Eocene into the Cretacious, is conclusive of the point we would make, that no great evolutional changes have taken place in the last two or three million years, and none are likely to take place in the next million years, except that the Sequoia gigantea may drop out, from the vandalism of man or the next glacial drift.
M. Ch. Martins, in his "Voyage Botanique én Norwege," says "that each species of the vegetable kingdom is a kind of thermometer which has its own zero." It may also be said to have its hygrometric and telluric gauges, or instruments to determine the necessary conditions of moisture and soil-constituents. When the temperature is below zero, the physiological functions of the plant are suspended, either in temporary hybernation or death. And so when the hygrometric gauge falls below the point of actual sustentation, the plant shrinks and dies; while, without the necessary conditions, it would never have made its appearance. There was nothing more imperative in the command for the earth to bring forth than the necessary conditions on which plant-life depended in the first instance, and still depends, as we have endeavored to show.
Dr. J.G. Cooper, in an interesting article prepared by him at the expense of the Smithsonian Institute, on the distribution of the forests and trees of North America, with notes and observations on the physical geography, climate, etc., of the country, after classifying, arranging, and tabulating the results of the various observations forwarded to that institution, indulges in the following general observations: "We have with a tropical summer a tropical variety of trees, but chiefly of northern forms. Again, with our arctic winters, we have a group of trees, which, though of tropical forms, are so adapted to the climate as to lose their leaves, like the northern forms, in winter. But, here, it must be distinctly understood, is no alteration produced by climate. Trees are made for and not by climate, and they keep their characteristics throughout their whole range, which with some extends through a great variety of climate." The italics are the authors, and we suppose he means by "tropical" and "arctic," the sub-tropical and sub-arctic.
In making his general observations, he had before him large collections of the leaves, fruits, bark, and wood of trees from all parts of the United States, including portions of Mexico, the Canadas and Alaska, and extending from the Atlantic to the Pacific. But one of the most important elements--in fact, the most important--is wanting in the tables before us, and that is, the elevation at which these thousands of specimens were obtained. So great an oversight as this should not have occurred, although it may not have been entirely Dr. Cooper's fault. He had his materials to work upon, and may have done the best that any one could with them. And yet it is just as important to know at what elevation a particular tree grows in its own plant zone, as to know whether it comes from a sub-arctic or sub-tropical region.
But this was not the comment we designed to make. Dr. Cooper labors, with most professional botanists, under the delusion that all our plants and trees originated in some one "centre of creation," at some period or other in time and place, and have been steadily spreading themselves outward from that centre until they occupy their present areas of distribution. We have no objection to his clinging to this superannuated faith and belief, if he derives any pleasure in flushing up these "traditional grasshoppers." But we have a right to insist that he shall be logical. He wants it distinctly understood that trees are made for, and not by, climate. Then his "centre of creation" should be everywhere, not a localized one. For he insists that no alteration can be produced by climate, but that the characteristics of each specific form are preserved throughout its entire range of distribution. But if these nomadic and migratory forms have wandered thus far from their centres of creation, it would seem that the trees had either adapted themselves to the climate, or the climate to the trees. But our Smithsonian systematizer will allow us neither horn of this dilemma. He insists that the trees were made for the climate, and that they have preserved their characteristic features during their entire ambulation upon the earth's surface.
With the change of a single monosyllabic predicate, this proposition is undoubtedly true. We have never heard that plants or trees were "made." They were ordered "to grow," or rather the earth was commanded to bring them forth, which is an equivalent induction. And the fact that they grow now, renders it absolutely certain that they grew at first, when "out of the ground made the Lord God to grow" every plant of the field, and every tree that is pleasant to the sight. We accept this genesis for the want of a better. And if Dr. Cooper will add to his climatic conditions, the hygrometric and other conditions necessary for the development and growth of his plants and trees, we will agree with him to the fullest extent of his novel position--that trees neither adapt themselves to the climate, nor the climate to the trees; although it is true that trees modify climate quite as much as they are modified by it. The true physiological formula is undoubtedly this:--Trees make their appearance in climatic and other environing conditions, and flourish, without material change in characteristics, so long as these conditions favor. Why they make their appearance is not a debatable question, except as we assume a preëxisting vital principle, and apply to its elucidation our subtlest dialectical methods. We are told that God commanded the earth to bring them forth, after his spirit (the animating soul of life) had moved upon the face of the depths--the chaotic and formless mass of the earth in the beginning. Plato has uttered no profounder or more comprehensive truth than this, with all his conceptions of Deity and the perfect archetypal world after which he conceived our own to be modeled. Our preference for the Bible genesis over the Platonic conception is, that it is vastly simpler and constitutes a more objective reality to the human soul. Besides, we find it true in fact, since the earth is constantly teeming with life, as if in obedience to some great primal law impressed upon matter by an infinitely superior intelligence to our own.--
"If this faith fail,
The pillar'd firmament is rottenness,
And earth's base built on stubble."