3S 6 THE POPULAR SCIENCE MONTHLY. 



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 ab- 

 sence 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 Palaeozoic there is a lack of any indica- 

 tions 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 23J, 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 mill- 

 ions 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 spe- 

 cies, 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, 



* Dana's u Manual of Geology," third edition, page 352, says : " If we draw any con- 

 clusions from the facts, it must be that the temperature of the arctic zone differed little 

 from that of Europe and America. Through the whole hemisphere and we may say 

 world there was a genial atmosphere " (and corresponding conditions as to actinic in- 

 fluence) "for one uniform type of vegetables, and there were genial waters for corals and 

 brachiopods." 



