EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 343 



throughout, such similarities are clearly no real evidence of relation- 

 ship of the plants which show them. A good example of this is seen 

 in Cactus and Euphorbia, two genera widely apart in the System of 

 Flowering Plants. Both of these genera show turgid succulence of 

 the stem and reduction of their leaf-area as an adaptation to con- 

 ditions of drought (Fig. 141). However similar to one another in 

 vegetative structure such plants may appear to be, the difference of 

 their floral characters shows that they are not really akin. More- 

 over, the succulent Cacti are characteristic of the American Con- 

 tinent and the Euphorbias of Africa. Thus both their floral structure 

 and their distribution suggest that their vegetative resemblance 

 indicates homoplasy. Similar arguments will apply to the climbing 

 habit by means of twining stems or tendril-like leaves : to the spinous 

 development of stems and leaves : to parasitism : to zygomorphy in 

 vegetative shoots and in flowers : to gamopetaly and epigyny : and to 

 many other familiar features that have been already noted in Flower- 

 ing Plants. Such characteristics cannot safely be held as evidence in 

 themselves of affinity of the plants which show them, since they may 

 appear independently in plants otherwise quite distinct. They are 

 often merely examples of Homoplasy. Such homoplasy, or parallel 

 development, is thus a famil ; ar fact in Flowering Plants. In the study 

 of the lower organisms the observer should be prepared to meet with 

 similar consequences of adaptation, resulting from a like accommoda- 

 tion to the same conditions of life in organisms not closely related by 

 Descent. He should be willing to agree that in them also apparent 

 similarity of characters may not be a sign of affinity, but only of 

 homoplastic adaptation. Especially will this be so for the vegetative 

 system, which is not so reliable a guide as the more stable organs of 

 propagation. All parts are, in point of fact, subject to adaptation 

 under the varied circumstances of life. Consequently it is not upon 

 single characters, which may be only homoplastic adaptations, but 

 upon the sum of all the characters, external and internal, that the affinity 

 of plants, whether high or low in the scale, is to be judged. This is 

 the broad base upon which a Natural System of Classification should 

 rest. 



Homology and Analogy. 



The fact of widespread homoplastic adaptation makes it necessary 

 to have some more reliable basis for the classification of the parts 

 of the Plant than mere comparison of their external form or even 

 of their internal structure. Such classification of parts must be based 



