176 NATURE AND MAN. 



ever, of any one species, must have been concentrated in their 

 first progenitors — a doctrine scarcely less monstrous than that of 

 the emboitement of the germs themselves, which were once supposed 

 to lie packed one within the other, like nests of pill-boxes. 



Now, as the process of physiological inquiry has been recently 

 bringing more and more clearly into view the dependence of all 

 Vital activity upon certain antecedent conditions, it has especially 

 established such a definite relation between the degree of this 

 activity and the amount of Heat supplied to the organism, either 

 from external or internal sources, as to make it clear that this 

 agent is much more than a mere stimulus or provocative to the 

 exercise of the vital force, and really furnishes the power that does 

 the work. It has been, in fact, from the narrow limitation of the 

 area over which physiological research has been commonly pro- 

 secuted, that this great truth has not sooner become apparent. 

 Whilst the vital phenomena of warm-blooded animals, which 

 possess within themselves the means of maintaining a constant 

 temperature, were made the sole, or at any rate the chief objects 

 of study, it was not likely that the inquirer would recognize the 

 influence of external heat in accelerating, or of cold in retarding 

 their functional activity. It is only when the survey is extended 

 to cold-blooded animals and to plants, that the immediate and 

 direct relation between heat and vital energy — as manifested in 

 the rate of growth and development, or of other changes peculiar 

 to the living body — is unmistakably evinced. 



All the facts and generalizations of Botanical Geography point 

 to the uninterrupted supply of a large measure of light and heat as 

 the source of the rich luxuriance and perennial activity of tropical 

 vegetation ; whilst the periodical declension of vegetative activity 

 which we observe in the trees and plants of the temperate zone, is 

 no less obviously due to the seasonal diminution in the supply of 

 these agents. So, again, the entire cessation of all manifestations 

 of vegetative life during the protracted intensity of an arctic winter, 

 is in striking contrast with the almost incredible rapidity of develop- 

 ment, which is observable under the unintermitted beams of the 

 summer sun. Now, there are certain annual plants, such as the 

 corn-grains, which will flourish under a considerable variety of 

 climatic conditions, and whose term of life is definitely marked 



