﻿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, some species of Smilax, etc. No other 

 colours have been met with, with one remarkable exception ; frag- 

 ments of a reed-like plant are found of a deep violet, staining the 

 surrounding clay mauve for a considerable distance." The shape, 

 the venation, 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 following fossil forms were mentioned as having been 

 determined with but little doubt. Feather and fan-palms, Dryan- 

 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 remind 

 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, tell us that in recurring periods, 

 each hemisphere, 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 versd. Whenever the area 

 became gradually colder, 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 



