Trans. N. Y. Ac. Set. 88 April 14, 



conditions, but all through the earlier ages there were no special floras 

 for different latitudes. One type prevailed everywhere. 



The marvellously luxuriant foliage of the Arctic trees has excited the 

 surprise of Lyell and others. 



It is a matter of common observation that plants receiving the full 

 intensity of the sun's rays have smaller leaves than their fellows of the 

 same species which are somewhat protected. It would seem as if 

 nature compensated for the inferior intensity of the solar rays by giv- 

 ing more surface to be acted upon. Since the intensity of the sun's 

 rays varies as the cosine of the latitude, it is evident that, while the 

 length of the day in Florida and Spitzbergen would be the same, with 

 the sun all the year on the equator, the intensity of the light in the 

 former would be almost twice as great as in the latter. Hence, if the 

 earth's axis was in fact nearly, or quite, perpendicular, and if, in some 

 way, the temperature was kept the same, we ought to look for greater 

 size of foliage — greater "luxuriance" — in very high latitudes than in 

 low. 



The existence of annual growth-rings in the early exogenous plants 

 is not proof of the existence of seasonal changes, for such rings may 

 be formed several times in a summer, or not all, or once in several 

 years. I have seen a hard and woody stem of Che7iopoditnn album, 

 less than four months from the seed, with eight well-formed rings. A 

 woody Phytolacca, according to Dr. Gray, makes at least twice as 

 many layers as it is years old. The Cycads require several years to 

 make one. The orange and lemon, in greenhouses where the tempera- 

 ture is kept uniform, form layers as regularly as do our forest trees. 

 The mangrove, which grows between high and low-water mark on the 

 sea-shore in tropical regions, forms well-marked rings. In its case there 

 is no seasonal change, either from warm to cold or from wet to dry. 

 Hence I infer that the presence of these layers, or growth-rings, is 

 not due to influences connected with the seasons, and, therefore, has 

 no bearing upon the question of the inclination of the earth's axis.* 



I think we may say that the teachings of geology are what they 

 would be if the earth's axis was in those times nearly or quite perpen- 

 dicular to the ecliptic, providing that in some way the temperature 

 was kept sufficiently high. 



Nor is it any argument against the axis having formerly been in that 

 position that no traces are found of such a cataclysm, when it changed 

 to its present obliquity, as would have attended an equal change in 

 the geographical position of the poles. The latter would have neces- 

 sitated a great overflow of the ocean, while a change of inclination, if 



* For a fuller discussion, see Am Jour. Sci., 1878, Article XIV., entitled, " Is the Exist- 

 ence of Growth-rings in Exogenous Plants Proof of Alternating Seasons ? " 



