4 INTRODUCTION 



unchanged; owing to the change of function which has taken place 

 such surviving features are physiologically valueless. An example 

 will serve to render the argument more convincing. 



In the genus Aloe, the leaf-margin is strengthened by thick-walled 

 hypodermal mechanical cells of palisade-like form; transitional stages 

 slmw that these palisade-stereides ai*e phylogenetically derived from 

 thin-walled chloropbyU-containing palisade-cells, that is to say, from 

 typical photosynthetic elements. Now the palisade-like form of cells 

 specialised t'ni' photosynthesis is closely related to their function; 

 next to the presence of numerous chloroplasts, in fact, it is their 

 niest important anatomico-physiological characteristic. This feature 

 is inherited unchanged when the photosynthetic cells of the leaf- 

 niargin of Aloe undergo the above-mentioned change of function, 

 because the retention of this characteristic does not interfere with the 

 newly assumed mechanical function. In respect of this new function, 

 however, the palisade-like form of the cell is a useless feature, since it 

 is by no means necessary that the mechanical cells employed for the 

 purpose of strengthening the leaf-margin should be elongated at right 

 angles to the surface of the leaf. As a matter of fact the protection 

 of leaf-margins against the danger of laceration is far more frequently 

 effected with the aid of fibrous strands, composed of cells that are 

 elongated in a direction parallel to the leaf-surface. 



A third category of functionless characters includes features which 

 occur in association with some adaptive structure, their existence 

 being determined by the particular mode of development of the 

 latter. Very frequently, namely, the development of an adaptive 

 arrangement involves the appearance of features which have no 

 intrinsic physiological value, but which arise incidentally in connection 

 with certain stages of a particular sequence of developmental processes. 

 When a board is planed in a joiner's workshop spirally rolled shavings 

 are produced; yet only a child would imagine these to be objects 

 prepared for a definite purpose. 



These useless correlative characters are of course not easy of 

 recognition, since the discovery of such features generally implies a 

 complete insight into the developmental mechanism of the adaptive 

 structure with which they are correlated. An illustration is furnished 

 by the stratification of thickened cell walls, which is so clearly 

 displayed by many mechanical cells. Here the thickness of the 

 membrane is the factor which is of physiological importance: the 

 stratified condition, on the other hand, is valueless from the mechanical 

 point of view, and merely represents an inevitable consequence of 

 the particular mode of thickening adopted by the developing cell 

 wall. 



