THE SHAPES OF LEAVES. 14<') 



di£Ferentiated in proportion as their relations to incident 

 forces become different. And here, as before, we see that in 

 each unit, considered by itself, the differences of dimension 

 are greatest in those directions in which the parts are most 

 differently conditioned ; while there are no differences be- 

 tween the dimensions of the parts that are not differently 

 conditioned.* 



* It T\-as by an observation on the forms' of leaves, that I was first led to tha 

 views set forth in the preceding and succeeding chapters on the morphological 

 differentiation of plants and animals. In the year 1851, during a countrv 

 ramble in which the structures of plants had been a topic of conversation with a 

 friend— Mr G. H. Lewes — I happened to pick up the leaf of a buttercup, and 

 drawing it by its foot-stalk through my fingers so as to thrust together its deeply- 

 cleft divisions, observed that its palmate and almost radial form was changed 

 into a bilateral one ; and that were the divisions to grow together in this new 

 position, an ordinafy bilateral leaf would result. Joining this observation with 

 the familiar fact that leaves, in common with the larger members of plants, 

 habitually turn themselves to the light, it occurred to mo that a natural change 

 in the circumstances of the leaf might readily cause such a modification of form as 

 that which I had produced artificially. If, as they often do with plants, soil 

 and climate were greatly to change the habit of the buttercup, making it 

 branched and shrub-like ; and if these palmate leaves were thus much over- 

 shadowed by each other; would not the inner segments of the leaves grow 

 towards the periphery of the plant where the light was greatest, and so change 

 the palmate form into a more decidedly bilateral form ? Immediately I began to 

 look round for evidence of the relation between the forms of leaves and the general 

 characters of the plants they belonged to ; and soon found some signs of con- 

 nexion. Certain anomalies, or seeming auomalies, however, prevented me from 

 then pursuing the inquiry much further. But consideration cleared up these 

 diificulties ; and the idea afterwards widened into the general doctrine here 

 elaborated. Occupation with other things prevented me from giving expression 

 to this general doctrine until Jan. 1859 ; when I published an outline of it in 

 the Medico-Chirurgical Ilmiiew, 



