342 BOTANY OF THE LIVING PLANT 



dispensed with its foliage leaves as a consequence of its parasitism. 

 An extreme case is seen in Rafflesia with its huge flower, while its 

 absorbent vegetative system has been reduced almost to the level 

 of that of parasitic Fungi (p. 227). Thus a simpler structure may 

 sometimes appear as a consequence of evolutionary changes, though 

 in the great majority of cases evolution leads to greater complexity. 

 Such considerations as these must be borne in mind while examining 

 and comparing organisms from an evolutionary point of view. Many 

 other precautions are also necessary before Lines of Descent can be 

 traced with any degree of probability. If we figure to ourselves the 

 whole plan of Descent of Plants as a highly ramified tree, or group 

 of trees, the great majority of living species would correspond only to 

 the distal twigs, while their connections downwards to earlier branches, 

 which represent their ancestry, are mostly wanting. At best they are 

 suggested perhaps by a few isolated, archaic forms, or vaguely sketched 

 in by occasional fossil remains of earlier time. It may even be a ques- 

 tion whether there is really any connection downwards to a single 

 trunk : for it is quite reasonable to suppose that the Evolution of the 

 Vegetable Kingdom may have been not monophyletic, that is, along 

 a single main line or phylum, but polyphyletic, that is, along a plurality 

 of lines. Notwithstanding the uncertainty on points even so impor- 

 tant as this, a belief in Evolution appears to be fully justified by the 

 facts. Origin by Descent should therefore form the constant back- 

 ground to any intelligent comparison of living Plants when taken in 

 progressive sequence, leading as it habitually would from those which 

 are relatively simple to those which are more advanced. 



Homoplasy. 



In Chapter XI. (p. 193) special modifications of form which lead 

 to successful life under special conditions have been designated 

 adaptations. There is reason to believe that they have arisen in the 

 course of Descent in relation to those conditions : in fact, that the 

 plants which show them have been adapted to their environment. If 

 two or more races of plants quite distinct from one another in Descent 

 have developed independently under similar conditions, and been 

 adapted thus to those conditions, a similarity of contour or of structure 

 may be expected to result. A parallelism of development, or as it 

 is called Homoplasy, would then be seen. Such similarities in distinct 

 races would be described as homoplastic. Having been distinct in origin 



