Editorial Cominciit. 121 
may have been distributed in a zonal manner, on the earth's 
surface, must have been seasonal fluctuations in the rainfall. 
Indeed, it is quite probable that this fluctuation was felt zonally 
on the earth's surface, and perhaps induced wet and dry sea- 
sons, or wet and dry belts that encircled the earth. At the 
present time, the seasons of tropical and low temperate lati- 
tudes are zcct and dry, rather than cold and hot. The wet sea- 
son is the season of growth and all exogenous vegetation adds 
to its diameter another ring. The 'dry season supervenes and' 
exogenous growth is stopped as effectually as if chilled by the 
cold of a northern winter. There may have been, therefore, 
consistent with Mr. Manson's theory, such alternations of wet 
and dry as to cause the development of the ringed trunk, even 
in Devonian time. 
It was at the opening of the, Cretaceous, however, that a 
great revolution took place. This was physical and organic, 
and was world-wide. After this revolution, exogenous plants 
were greatly multiplied. So great was the approach toward 
modern forms of trees, that most of the exogenous arboreal 
genera now extant, began then their existence. If exogenous 
growth requires the agency of direct sunlight, it would be 
necessarv to allow th&t the sun's light then burst upon the sur- 
face of the planet in its virgin eft'ulgence. But these genera, 
while flourishing under the alternations of winter and summer 
in the north, are found also in the south, where the seasons al- 
ternate between wet and dry. Since tropical forms of planets 
flourished from the equator to the poles, even into the Tertiary, 
and since exogeneous plants flourished throughout from the 
commencement of the Cretaceous to the present, it is evident 
that exogenous growth is not wholly dependent on the alterna- 
tions of summer and winter, such as now characterize the north 
temperate zone. But some cause became operative which ex- 
pelled tropical plants from the poles. Whatever that cause was, 
it did not determine the life and distribution of exogcns. 
The foregoing objections may require some modification in 
the application of the theory of Mr. Mason, but they are ap- 
parently not fatal to it. So far as can be judged by the writ- 
er, the cause of the ice age appealed to by the new theory, has 
a large and promising element of truth. It has been cautiously 
accepted by some geologists and cordially received by some 
