CH. Ill] THE EFFECT OF CLIMATE 123 



observations go) whether the essential factor be a fluctuation of 

 temperature or an alternation of moisture and aridity ; but the 

 character of the Arizona climate, and the known facts of recent 

 years, encourage the belief that the latter is the more direct and 

 more important factor. 



It has been often remarked that our common European trees, 

 such for instance as the elm or the cherry, tend to have larger 

 leaves the further north we go ; but in this case the phenomenon 

 is to be ascribed rather to the longer hours of daylight than to 

 any difference of temperature*. The point is a physiological one, 

 and consequently of little importance to us here f ; the main point 

 for the morphologist is the very simple one that physical or 

 climatic conditions have greatly influenced the rate of growth. 

 The case is analogous to the direct influence of temperature in 

 modifying the colouration of organisms, such as certain butterflies. 

 Now if temperature affects the rate of growth in strict uniformity, 

 alike in all directions and in all parts or organs, its direct effect 

 must be limited to the production of local races or varieties dift'ering 

 from one another in actual magnitude, as the Siberian goldfinch 

 or bullfinch, for instance, differ from our own. But if there be 

 even ever so little of a discriminating action in the enhancement 

 of growth by temperature, such that it accelerates the growth of 

 one tissue or one organ more than another, then it is evident that 

 it must at once lead to an actual difference of racial, or even 

 "■ specific " form. 



It is not to be doubted that the various factors of climate 

 have some such discriminating influence. The leaves of our 

 northern trees may themselves be an instance of it ; and we have, 



* It may well be that the effect is not due to light after all ; but to increased 

 absorption of heat by the soil, as a result of the long hours of exposure to the sun. 



t On growth in relation to light, see Davenport, Exp. Morphology, ii, ch. xvii. 

 In some cases (as in the roots of Peas), exposure to hght seems to have no effect 

 on growth; in other cases, as in diatoms (according to Whipple's experiments, 

 quoted by Davenport, n, p. 423), the effect of light on growth or multipHcation 

 is well-marked, measurable, and apparently capable of expression by a logarithmic 

 formula. The discrepancy would seem to arise from the fact that, while light- 

 energy always tends to be absorbed by the chlorophyll of the plant, converted into 

 chemical energy, and stored in the shape of starch or other reserve materials, the 

 actual rate of growth depends on the rate at which these reserves are drawn on: 

 and this is another matter, in which light-energy is no longer directly concerned. 



