12 
follow the same law over the whole earth, being twice or 
three times as much in one country as another : these differences 
not being in constant relation either with the latitudes or 
longitudes of places. In fact, they are variations of volcanic 
heat, and change their position as the scenes of volcanic 
action undergo change. Again, if subterraneous heat con- 
tinued through a polar winter of two or three months' dark- 
ness, ^plants would be destroyed. Mr. ~R. Hunt, in recent 
experiments on the action of the sun in the growth of plants, 
proves that the fixation of carbon is due to the actinic 
rays, and therefore the growth of plants is in proportion to 
the quantity of light. He informs Captain Drayson that he 
tried during nine years, and could not, without the light of 
the sun, produce by heat any of the tropical plants. Pro- 
fessor Lindley dwells upon this point as one of great diffi- 
culty, and supposes that the earth's axis of rotation has been 
changed. " Plants," he affirms, " cannot be retained in 
" darkness, even for a week, without serious injury, unless in 
" a torpid state, (i.e. from cold) and if exposed to heat and 
" moisture they cannot remain torpid, but will grow, and 
" therefore must perish. If then (says he) a high tempera - 
" ture and consequent humidity prevailed at that period, 
" when we know the arctic seas were filled with corals and 
" large multilocular shells, how could plants of tropical forms 
" have flourished ? Is not the bright light of equatorial regions 
" as indispensable a condition of their well-being, as the sultry 
" heat of the same countries . ? "* Lyell, in answer, says that 
all astronomers are agreed that speculations on the change in 
the axis of rotation of the earth are inadmissible, but I reply 
that a change in the direction of the axis is not. Brongniart 
asserts that the light and heat now enjoyed by the equator, 
or " the hottest parts of the globe" are inadequate and insuf- 
ficient for the production of the enormous size, as well as 
* Lindley's Lectures IV. on Fossil flora. 
