Page:Popular Science Monthly Volume 29.djvu/373

Rh summer and winter. They may occur several times in a summer, or not at all, or once in several years, or where there is absolutely no change of seasons. I have seen a hard and woody stem of Chenopodium album, not more than four months from the seed, in which were eight well-formed rings. Dr. Gray says there is "a woody Phytolacca which makes at least twice as many layers as it is years old," and that cycads require several years for one layer. The orange and lemon, in green-houses, where seasons can hardly be said to be known, form ring's as well defined as those of our forest-trees. On the Amazon, as may be seen in a collection of woods now in Vassar College, the rings are very apparent in some species, while in others equally exogenous none can be seen. The mangrove, which grows in the tropics on the sea-shore between high and low water mark, where by no possibility can there be any annual change either in temperature, or from wet to dry, has the rings well developed. These facts suffice to prove that the existence of growth-rings is independent of the existence of seasons.

I think it must be admitted that the teachings of geology are in harmony with what would have been the climatic conditions in high latitudes if the axis of the earth was then perpendicular (or nearly so) to the ecliptic, provided that in some way the temperature could have been kept up sufficiently. And, if there be anything in the influence of environments, the lack of results corresponding to days and nights so different as those, e. g., of Spitzbergen and Florida, is evidence that the days and nights in those countries did not differ then as they now do. If there was no such difference, the earth's axis then did not incline as it does at the present day.

The tilting of the earth, or, in other words, changing the direction of its axis—if gradual—would occasion no perceptible disturbance. Hence no conclusion is to be drawn from the absence of traces of such a cataclysm as would have attended a change of the geographical position of the poles. The latter, however slowly brought about, would have necessitated a change in the position of the equatorial protuberance, or, if the crust was too rigid for that, a change in the ocean sufficient to overwhelm the land. The only possible effect of an increase in the obliquity of the axis would be an increase in the length of the days and nights in high latitudes followed by corresponding climatic changes. These would have registered themselves in the plants and animals of high latitudes, while near the equator the effect would be scarcely perceptible. Days and nights in low latitudes would be only slightly affected, consequently animal and vegetable life would continue as before. It is corroborative of such a tilting, that the plants and animals in high latitudes, which, till near the end of the Tertiary,