EVERGREEN AND DECIDUOUS TREES, ETC. 
G5 
during prolonged darkness it is re-absorbed (see Sachs), but 
if it is stored away when formed in separate cells, as in seeds, 
tubers, &c., it continues unchanged. 
Reviewing and collating these facts I suggest the follow¬ 
ing provisional theory :—• 
Whether any group or botanical order of trees or shrubs 
is normally evergreen or deciduous depends upon the climatal 
conditions under which it was evolved, differentiated and 
established as a distinct group ; and the character then 
assumed is perpetuated by heredity through its later ramifi¬ 
cations, although the climatal conditions may have changed 
considerably. But an order which was originally and is still 
normally evergreen may have evolved certain genera or 
species under later and much altered conditions, which 
assumed the deciduous character, and vice versa. 
An evergreen order is one which was evolved under a 
climate in which the change of seasons was but slightly 
marked. A deciduous order, on the contrary, originated under 
a climate in which the conditions of winter and summer 
differed widely. 
The only extensive region of the earth, as it now is, in 
which the seasons are but slightly marked, is the tropical 
zone, and this state of things has existed probably for at 
least 100,000 years. But in earlier geological epochs, there 
is good reason to believe that the climate of the whole sur¬ 
face of the earth was much more equable than it is now, much 
more resembling the conditions of our present tropical zone. 
Evergreens, therefore, probably originated either in these 
early epochs, or else in the neighbourhood of the tropics. Of 
existing trees and shrubs, the oldest group is undoubtedly the 
Coniferce. This order originated at least as far back as the 
Carboniferous period, and is normally evergreen, as might 
have been expected, with a few deciduous forms probably of 
late development. The earliest group of Angiospenns, the 
truly exogenous trees and shrubs, occurs in the Wealden 
formation, just before the opening of the great Tertiary 
epoch ; and this group consists almost entirely of the curious 
evergreen order of Proteacea, now chiefly confined to the 
Cape of Good Hope and Australia. 
In the lower Eocene strata are found, in addition to the 
Proteads, the evergreen Figs and Laurels ; while the London 
clay yields also Anonas, Acacias, and Palms, all evergreen 
groups. Not till the Miocene period do such distinctly 
deciduous genera as the Willows, Poplars, Maples, Hazels, 
Birches, and Beeches make their appearance, and still these 
are in the minority, the forests evidently consisting mainly 
