412 • MEMOIRS OF THE AMERICAN ACADEMY, 



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development in the progress of the* leaf along the line of its highest ascent 

 in development, from the segmentations we have supposed in the twisted frond 

 we would soon arrive at the steps already familiar in the principles of vege- 

 table morphology. In these we have the same law of * segmentation or sepa- 

 ration of parts, and the same successive relations of genetic and adaptive 

 characters. What was produced for one purpose becomes serviceable to 

 a new one; and in its capacity as a merely genetic character, or as an 

 inherited feature, becomes the basis for the acquisition of new adaptations. 

 Thus the fibrous structure, at first useful in sustaining the softer tissues of 

 the leaf, becomes the means of a longitudinal development of it, and its more 

 complete exposure to light and air by the growth of the foot-stalk. This stalk 

 acquires, next a new utility in climbing-plants to which it becomes exclusively 

 adapted in the tendril. The adaptive characters of the tendril are its later 

 acquisitions. Its genetic characters, such as its position on the stem, and its 

 relations to the leaves, become useless or merely inherited characters. The 

 contrast of genetic and adaptive characters appears thus to have no absolute 

 value in the structure and lives of organisms, but only a relative one. The 

 ■st are related principally to past and generally unknown adaptations; the 

 second to present and more obvious ones. 



In accordance with this law I have supposed that the general features of 

 the two types of leaf-arrangement, for which no present utilities appear in the 

 lives of the higher plants, were nevertheless useful features in former conditions 

 of vegetable life. The more special features of these 'arrangements should not, 

 from this point of view, be regarded as derived one from another, much less 

 from the typical or unique form of the theory of Phyllotaxy. In one sense 

 they may, indeed, be said to be derived from this form, at least some of 

 them; yet not from it as an actually past form or progenitor, but rather from 

 the utility which it represents in the abstract. I have, however, pointed out 

 that another utility, shown in the simpler cyclic arrangements, has an equal 

 claim to this spiritual paternity. The actual forms of the spiral arrangements 

 in leaves should, therefore, be regarded as forms independently selected, and as 

 selected on the two principles of utility, which we have considered, out of a 

 very large variety of original forms. We have seen that even those forms 

 which survive include almost all possible ones that could be distinguished; 

 though the more prevalent ones are at present in the minority. We have 

 also seen that the latter fact, and the more frequent occurrence of inferior 



