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.