Page:Popular Science Monthly Volume 29.djvu/370

356 The fact of their continued existence, not dwarfed and scanty, but with greatness of size and luxuriance of growth, seems to indicate that there could not have been a total cessation of solar heat for months in winter, and an uninterrupted influx for months in summer. In other words, the evidence of plants and animals points to the absence of present long days and nights.

However it may be, as late as the Tertiary, geologists are agreed that at least to the end of the Palæozoic there is a lack of any indications of zones of climate, or, to put it in another form, that there is evidence only of evenness of climate.

I next inquire, What is indicated as to the length of the arctic day by the effects of light upon plants?

In all discussions of these curious facts—at least so far as I have seen—no attention has been paid to the effect upon vegetable and animal life, of the great difference between the length of the days and nights in high and low latitudes, even though the temperature were kept up. In Spitzbergen, for example, the sun shines uninterruptedly for four months, and for an equal time its rays are cut off, while in tropical regions a day of twelve hours is followed by a night of the same length. In the temperate zone the day is at most but a few hours longer. If the earth's axis in preglacial times was inclined 23°, the same inequality prevailed then. Light is as necessary to plant-life as heat, and, in respect to the character of the polar day, its evidence is more important, since light is affected only by the inclination of the earth's axis. The flow of the Gulf Stream, the lay of the land, or the relative amount and arrangement of the land and water—matters of great moment when considering questions of temperature—have no effect whatever upon the length of the day, or, in other words, upon the mode of light distribution.

Mr. Darwin and his followers have called attention to the influence of environments in destroying old species and in the production of new. In view of all that they have established, it seems incredible that species identically the same could have lived and shown luxuriant growth, say in Spitzbergen and Florida, through thousands and millions of years, unaffected by such difference as now exists in the length of the days and nights. The arguments against the reality of such a difference become stronger when we reflect that in both high and low latitudes there were from period to period enormous changes in species, old ones passing away and new ones appearing, not once, nor twice, but a great many times, and yet at each epoch the new species,