934 ORIGIN OF SPECIES, 



understanding by metamorphosis the adaptation to various physiological purposes of 

 morphologically equivalent members. The diversity in the physiological development 

 is related to the conditions of life of the plant ; and to this extent Metamorphosis is 

 synonymous with what we have here termed Adaptation, and which has also been 

 described as Accommodation. When we speak of Purpose in the structure of a 

 plant, we mean in fact nothing more than that the form and the other characters of 

 the organ are adapted to the conditions of its life, which may be at once inferred from 

 the very fact of the survival of the plant in the struggle for existence. The terms 

 Purpose, Adaptation, and Metamorphosis express therefore the same thing, and may 

 be used as synonymous, as we have already repeatedly done^. 



For the discussion of the questions to be treated of in the following paragraphs 

 it is important to have as clear a conception as possible of the relationship of adapt- 

 ation to the morphological nature of the organs, and of the great constancy of 

 morphological characters and the infinite diversity of metamorphosis; for this re- 

 lationship can be explained by no other theory than that of descent. 



In its most general features the relationship of adaptation to the morphological 

 nature of organs is manifested in the fact that all the various morphological members 

 perform the most different functions and in an infinite variety of ways; in other words, 

 that the morphological nature of the parts of a plant is not directly determined by 

 their function, nor is the function of an organ determined directly by its morpho- 

 logical nature. Thus, for example, trichomes sometimes take the form of a pro- 

 tective envelope (mostly in buds), sometimes of glands, sometimes of absorptive 

 organs (as root-hairs), sometimes of asexual organs of reproduction (as the sporangia 

 of Ferns), &c. The leaves again are usually organs of assimilation containing chlo- 

 rophyll ; but they may also be employed as protective envelopes to winter-buds (in 

 most of our native woody plants), as reservoirs for reserve food-materials (in the 

 seedlings of flowering plants and in bulbs). In the majority of plants the leaves 

 bear the sporangia: in flowering plants the flowers are composed of peculiarly 

 metamorphosed leaves. In many slender-stemmed Angiosperms the leaves are 

 transformed into tendrils, in order to raise up the slender stem and fix it to neigh- 

 bouring supports ; the leaves of Nepenthes produce at their apex an appendage 

 which forms a pitcher provided with a moveable lid and filled with the fluid 

 which it itself secretes ; some of the leaves contained in flowers are developed into 

 nectaries and then perform the function of glands ; not unfrequently they are trans- 

 formed into hard woody spines ; in other cases they are sensitive to irritation, 

 contractile, and so forth. The parts of the axis are scarcely less varied in their 

 development ; sometimes they cling round upright supports ; sometimes they are 

 woody and able to retain themselves in an erect position ; sometimes they are slender 

 swaying branches, or thick fleshy succulent masses {Cactus), or round tubers filled 



^ Many recent writers seem to be almost too anxious to avoid the use of the word ' purpose,' 

 because it seems to suggest antiquated teleological views. The word 'useful' which they would 

 substitute for ' purposive ' has also a teleological significance in human affairs. If every word 

 which has once been used to express an incorrect theory is to be discarded, the resulting diminu- 

 tion of the vocabulary would soon produce an evident impoverishment of the language. The 

 mission of science is not to explain and alter words, but the ideas to which they correspond. Ought 

 we to give up the use of the word ' root ' in Botany, because it formerly bore a very different 

 meaning from that which is now attached to it ? 



