86 SCIENCE PROGRESS. 



sation at which he arrived. For what is this evidence ? It 

 consists, in the first place, of a careful comparative study of 

 the appendicular organs, noting the various forms which they 

 assume in different parts of any one plant as well as in 

 different plants, with special reference to those cases, to be 

 found here and there, where transitional forms occur. But 

 external resemblances are by no means conclusive ; it would 

 be easy to cite numerous examples of close external simi- 

 larity between organs as distinct as stems and roots, stems 

 and leaves, and roots and leaves ; in fact several examples 

 of the kind are now before you. Then again he relies upon 

 monstrosities, such, for example, as stamens which have 

 assumed the character of petals, as sepals that of ordinary 

 green leaves. But teratological evidence is quite incon- 

 clusive, though it is suggestive ; for when any part deviates 

 m form from the normal, it does not follow that the form 

 which it assumes is a primitive form indicative of its 

 essential nature ; there is no conclusive reason for regarding 

 such a case as one of reversion. What was lacking in 

 Goethe's observations was largely supplied by Wolff's elabo- 

 rate developmental studies ; for the study of development 

 is the only sure means of determining the nature, and thus 

 also of establishing the homologies, of the parts of any living 

 organism. 



In reviewing the doctrine of metamorphosis in plants 

 as it was left by Wolff and by Goethe, we cannot fail to 

 perceive that, great as was the advance they had made, 

 it was far from complete. In the first place, its applica- 

 tion was confined to one set of organs, the appendicular 

 organs, neglecting others of equal importance, such as the 

 stem and the root. And even with regard to the appendic- 

 ular organs much was left to be desired. They tell us that 

 all these organs are " leaves," and that the character common 

 to them is their mode of development from the growing- 

 point of the stem. They further tell us that the organs 

 of the flower are "modified leaves". But the question 

 remains, of what leaf or leaves are they the modifications ? 

 To this question two alternative answers have been offered. 

 The one which we may distinguish as the concrete, 



