160 MORPHOLOGICAL DEVELOPMENT. 



differentiated in proportion as their relations to incident 

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

 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 was by an observation on the forms of leaves, that I was first led to 

 the views set forth in the preceding and succeeding chapters on the mor- 

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

 a country ramble in which the structures of plants had been a topic of con- 

 versation 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 tc 

 grow together in this new position, an ordinary 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 me that a natural change in the circumstances of the leaf might 

 readily cause such a modification of form as that which I had produced arti- 

 ficially. 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 overshadowed by one another ; 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 belong to ; and soon found some signs of connexion. Certain 

 anomalies, or seeming anomalies, however, prevented me from then pursuing 

 the inquiry much further. But consideration cleared up these difficulties; 

 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 Review. 



