Mabch 1, 1907] 



SCIENCE. 



353 



rainfall (which later became the arid belts) ; 

 and thus gradually the zonal solar regime was 

 established. 



Such are main features of Hansen's theory, 

 the details of which have been elaborated in 

 his published treatise on the evolution of cli- 

 mates and other papers, and the substance of 

 which will be presented to this body by the 

 author himself. In my view it is not a con- 

 ception to be lightly set aside, for whatever 

 evidences of former glaciations may have been 

 observed, there has not appeared in former 

 geologic history anything resembling in mag- 

 nitude the pleistocene glaciation, the scattered 

 remnants of which are even now in gradual 

 retreat under our eyes. The observed evi- 

 dences of glaciation in former geologic ages 

 do not appear to be of such extent, or to be 

 accompanied by detrital deposits indicating a 

 continental extent of glaciation; they are ap- 

 parently such as might be produced at any 

 time by either the upthrusting of mountain 

 chains, or by wider, epeirogenie elevations of 

 the surface. Thus far, it seems as though 

 there had been but one distinctively glacial 

 epoch of world-wide importance and extent; 

 and that nearly contemporaneous with the ap- 

 pearance of man upon earth. 



It has been asked how the early floras and 

 faunas could have existed and developed 

 under the perpetual cloud assumed by Man- 

 son's theory to have covered the earth prior 

 to the establishment of the solar climate, 

 toward the end of the glacial period. In 

 answer to this it may be suggested, apart 

 from the fact that even at the present time 

 the average cloudiness of the sky is estimated 

 at 60 per cent., that the earlier floras con- 

 sisted almost exclusively of plants whose an- 

 alogues or evolutional successors, such as ferns 

 and horse-tail rushes, vegetate preferably in 

 dense shade, even in cloudy climates; and the 

 extreme succulence of the carboniferous flora 

 is sometimes approached when in certain cli- 

 mates, under unusally rainy seasons, such 

 plants grow to maturity almost without a ray 

 of sunshine. The plants growing under the 

 canopy of primaeval forests, in perpetual twi- 

 light, show how easily vegetation adapts itself 

 to such conditions. In later periods, as the 



cloud-envelope brightened, the higher orders 

 of plants, now preferably basking in sunshine, 

 had opportunity to develop to their present 

 prominence. But it is notable that the pres- 

 ent forms of peculiarly arid-region plants, 

 which are specially adapted to hot sunshine 

 and dry air, are absent from any of the fossil 

 forms thus far reported. They clearly had no 

 raison d'etre until the cloud-veil was dissi- 

 pated by the sun.* 



There seems to be as little difficulty in 

 assuming the animal creation to have been 

 tolerant of, or adapted to, a sunless existence. 

 Not to speak of our present nocturnal and 

 deep-sea faunas, the adaptability of the pupil 

 of the eye now existing provides all needful 

 conditions so far as vision is concerned; and 

 the great wide-open orbs of the ichthyosaurs 

 suggest ready adaptation to dark days. Here 



' Chamberlin (Science, October 26, 1906) claims 

 that the existence of palisade cells in plants of 

 paleozoic age proves the existence of arid-region 

 plants at that time. But palisade cells as such 

 depend much less upon climatic factors than upon 

 leaf-texture and botanical relationship. It is only 

 the presence of several tiers of such cells beneath 

 the epidermis of the upper leaf-surface that con- 

 stitutes such presumptive proof; witness the exist- 

 ence of abundant palisade cells in firm-leaved 

 ferns that are at home in the deepest shade, right 

 alongside of others which show no sueh tissue; 

 as well as the abundant palisade tissue in the 

 leaves of the shade-loving Pyrolaeese and other 

 ericaceous plants, of Tinea minor, Myosotis palus- 

 tris and thousands of other shade plants. More- 

 over, saline soils cause xerophytic structure and 

 growth; which, therefore, should not surprise us 

 if found in coal plants. The very generally clayey 

 (fire-clay) nature of the substrata of coal beds 

 plainly suggest that the coal-forming flora was 

 one of sioamp plants, and not xerophytic or even 

 upland, as suggested by Chamberlin. So far as I 

 am aware, no plants showing the well-known ex- 

 treme provisions against drying-out, such as we 

 find in the cactus and others, have been found 

 among the fossils of even the late Tertiary. On 

 the other hand, the fauna of the Permian, belong- 

 ing chiefly to the Brachiopod and Cephalopod 

 orders, indicates a warm temperate or tropical, 

 not a frigid temperature of the seas, such as is 

 shown by the marine fauna of the Pleistocene 

 ;ial epoch. 



