THE USESvAND ORIGIN OF THE ARRANGEMENTS OF LEAVES IN PLANT*. • 413 





m 



forms among fossil plants, are almost the only grounds on which the inductive 

 foundation of the theory of Phyllotaxy could be regarded as well established. 

 On these grounds, and on this foundation, I have sought by hypothesis to 

 reconstruct the continuity of higher and lower forms in vegetable life; and 

 through this to find the origin of the principal types of arrangement 



leaves. The speculation lies wholly within the limits prescribed for legitimate 

 hypothesis in science. It does not assume utilities in themselves unknown, but 

 assumes only unobserved or unknown applications of them, and raises to the 

 rank of essential properties relations of use, which, at first sight, appear to be 

 only accidental ones. Attention may be claimed at the least for it 

 illustration of the method by which the principle of Natural Selection is to 



be applied as a working hypothesis in the investigations of general physiology 

 or physical biology. 



Many features in the structure of leaves, not relating to their arrangements, 



fall beyond the proper province of this inquiry, but equally illustrate the rela- 

 tive nature of the distinction between genetic and adaptive characters. The 

 general character common to all leaves and leaf-like organs has an obvious 



is an 



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utility with reference to the function of nutrition. Some special modifications 

 have the purposes of defence, as in the thorn ; of mechanical support, as in 

 the tendril; and of reproduction, as in the parts of the flower. But the vast 

 variety of forms which leaves and the parts of flowers present do not suggest 

 any obvious uses. On the theory of adaptation they would naturally be referred 

 to a combination of adaptive and inherited features. A fixed proportion 

 between the two principal tissues in a plant due to some past utility may 

 without being changed, become adapted to new external relations, or to new 

 physiological conditions, through various arrangements of them in the structure 

 of the leaf; and this would give rise to a great variety of forms. The forms 

 of notched and sinuated leaves are referable to that process of segmentation 

 and reduction in leaf-expansions, which we have seen to be so important a 

 process in the derivation of the higher plants. But another principle of utility 

 comes into play in the ' lives of the higher plants, similar to that which 

 appears to be the origin of some of the more conspicuous external characters 

 of animals, namely, what produces distinguishableness and individuation in an 

 animal race. No doubt the laws of inheritance and Natural Selection account 

 for much of the character of individuality in races, or for the fact that varia- 

 tion has a very limited range compared to the differences between species, so 

 vol. ix. 56 



