402 G. R. Wielai\d — Polar Climate in Time. 



manner of terrestrial reception of the solar radiant energy. 

 Climate, altlioiigli following a fairly fixed trend, in itself sub- 

 ject in so far as tLe globe is concerned to an evolutionary 

 com*se, is at all times in given localities subject to the acci- 

 dence of countless movable conditions. 



Thus it is tliat in any attempt to reach a general idea of the 

 course of life on the globe, the scene of its origin, the location 

 of dispersion centers, and the more active factors in the chang- 

 ing of organisms and evolving of new species, it at once 

 becomes necessary to consult first of all the astronomical and 

 physical records. Secondly, it becomes a subject of the highest 

 interest when we are able to consult both the physical and 

 biologic record in those later periods of the earth's history 

 where these overlap. Each then supplements the other. In 

 taking a glance, however, at the subject of climate in time, 

 and particularly polar climate, I shall, leaving the considera- 

 tion of the pre-fossil period to the geo-phj'sicist, mention but 

 briefly several associated theories of an astronomical or phys- 

 ical nature, and then confine myself mainly to biologic data. 

 And I hold that the evidence taken in its entirety indicates as 

 most probable the polar origin of life, and the development 

 throughout more especially later geological time of the great 

 groups of animals and plants markedly possessing invasive 

 power, mainly in boreal regions. Secondarily these northern 

 forms dispersed themselves southward over the Y-shaped con- 

 tinents, and there appears to be but little likelihood that aus- 

 tral forms ever played as conspicuous a part in the great faunal 

 and floral movements of the past. In a word, that the great 

 evolutionary Schauplatz was boreal is possible from the astro- 

 nomical relations, probable from the physical facts, and ren- 

 dered an established certainty by the unheralded synchronous 

 appearance of the main groups of animals and plants on both 

 sides of the great oceans througliout post-Paleozoic time. 

 Moreover, the eflicient cause of this origin in high latitudes of 

 hardy and effective colonizers is to be sought for in the vicissi- 

 tudes of polar climate as compared with the more equable and 

 static conditions of the tropics. 



Climate in time, or geological climate, may be considered as 

 the resultant of two sets of factors, both of wliich are subject 

 to an evolutionary process which is in the one set more or less 

 irregular and subject to cataclysm, and in the other more purely 

 diflPerential or secular. The first set includes chiefly altitude, 

 emergence and stability of the land masses, deflection of the 

 winds and tides, and probably reception of meteoric material. 

 The main possible differential factors of geological climate are: 

 Change in obliquity of the ecliptic, axial fixity, internal heat 

 reaching the surface, rate of solar radiation, length of day, 



