On the Tropical Forests of Hampshire. 
25 
and the succulent leaves are always of light colour, as in the leaves 
which we suppose to be fig, souie species of Smilax, etc. No other 
colours have been met with, witli one remarkable exception ; i'rag- 
ments of a reed-like plant are found of a deep violet, staining the 
surrounding clay mauve for a considerable distance.” The shape, 
the veuation, and character of the margin of the leaves being the 
points by which comparisons are made with the leaves of trees now 
existing, were described at some length, and the difficulties of 
successfully making the comparisons were referred to. Among 
others the followiug fossil forms were mentioned as having been 
determined with but little doubt. Feather and fan-palms, Drynn- 
dra, beech, maple, Azalea, laurel, elm, acacia, aroids, cactus, ferns, 
conifers, Stenocarpus, and plants of the pea tribe, together with 
many others. “This question may perhaps have presented itself 
to your minds — how is it possible that the tropical forms of which 
we have spoken, such as the palm, aroids, cactus, etc., could have 
grown alongside of the apparently temperate forms, such as the 
oak, elm, beech and others ? Time does not allow that I should go 
at any length into the explanation of this; but I may just remiml 
1 you that in the long geological record of the beds found in England, 
there are to the geologist unmistakable indications of many changes 
in climate. Further, astronomers, having calculated the path of the 
revolution of the earth in ages past, teil us that in recurring periods, 
each hemispliere, northern and Southern, has been successively sub- 
ject to repeated cyclical changes in temperature. There have been 
for the area which is now England many alternations of long periods 
of heat and cold. Whenever the area became warmer, the descen- 
dants of semi-tropical forms would gradually creep further and further 
north, whilst the descendants of cold-loving plants would retreat 
from the advancing temperature, vice versa. Whenever the area 
became gradually cokler, the heat-loving plants would, from one 
generation to another, retreat further and further south, whilst the 
cold-loving plants would return to the area from which their 
ancestors had been driven out. In each case there would be some 
lingering remnants of the retreating Vegetation (though perhaps 
existing with diminished vigour) growing alongside of the earliest 
arrivals of the incoming Vegetation. 
Such is a possible explanation of our finding these plant-remains 
commingled together. It must, too, be borne in mind that it is not 
so much the mean temperature of a whole year which affects the 
possibility of plants growing in any locality, as the fact of what are 
the extremes of summer and winter temperature. For example, one 
place may have a mean winter temperature of 50°, and a summer 
one of 70° ; whilst another place might have a mean winter tem- 
perature of 20° and a summer one of 100°, and yet both have a mean 
annual temperature of 60°. 
In Cornwall the maiden-hair fern grows in sheltered localities, 
because the winter temperature never sinks to the point that would 
cause its destruction. Again, at that most charming spot in the 
west of Ireland, Glengariff, the Arbutus still forms an abundant 
