No. 464.] STUDY OF THE SALICACEE. 519 
diversification of the Cretaceous flora must have been initiated 
at correspondingly early periods, though the general succession 
of types of which we have knowledge, would indicate that it did 
. not gain full expression until about the time of the Middle 
Cretaceous. This diversification was no doubt the natural 
response of vegetation to the profoundly changed physical con- 
ditions, chiefly climatic, which followed the close of the Carbon- 
iferous age and resulted in a diminution of the carbon dioxide 
of the atmosphere to about one-twentieth of its original volume 
with a consequent reduction of atmospheric density ; a general 
purification of the atmosphere whereby it became more readily 
penetrated by the sun's rays; a lower and less equable temper- 
ature with more localized climates, and conditions which, as a 
whole, were more favorable to a more varied mesophytic type of 
vegetation. Under such circumstances, involving as they did 
a more abundant supply of oxygen, the general activities of 
growth became greater; the transformation of carbon dioxide 
into assimilable products became proportionately larger; in 
response to the more favorable conditions of light and air, the 
foliage became broader and acquired an increased functional 
capacity ; and with the augmentation of capacity for the storage 
of energy thus made possible, there were increased possibilities 
of, as well as strong tendencies toward diverse development in 
which special mutations no doubt played an important part, in 
response to even comparatively slight variations in environmental 
conditions. From this point of view it is not difficult to under- 
stand that among the numerous offshoots from the main line of 
descent there would be very varying degrees of activity, and it 
is readily conceivable that plants of a low phylogenetic position 
might be held in numerical abeyance for a long time, while 
others, originating at a higher level but more favorably situated 
and more capable of responding to their environment, might at 
once outdistance them in number of species or of varietal forms. 
Under such circumstances the geological succession might lead 
to erroneous conclusions as to the true phylogenetic sequence. 
Such a view is suggested and supported by a comparison of 
many well known examples among existing species. In this 
way it would be possible for a relatively superior type of organ- 
