Trans. IV. Vs AG: et. 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 cf 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 seena hard and woody stem of Chenopodium 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?” 
‘ 
