130 Development of the Natural System under [BOOKI. 



is perceived, without any attempt to give any account of it 

 in the detail, we have what has been called habitual relation- 

 ship ; but it is the task of the doctrine of symmetry to resolve 

 this likeness of habit into its elements, and to explain its 

 causes. Without this study of symmetry it may easily happen 

 that two different kinds of symmetry may be supposed to 

 be alike, because they seem outwardly alike to our senses, 

 just as forms of crystals of different systems may be con- 

 founded together for want of careful examination ; the chief 

 thing is to know the plan of symmetry in every class of plants, 

 and the study of this is the foundation of every theory of 

 natural affinities. But success in this study depends on the 

 certainty with which organs are distinguished, and the dis- 

 tinguishing them must be independent of changes of form, 

 size, and function. He then shows that the difficulties in 

 the morphological comparison of organs, or, as we should 

 now say, in the establishing the homology, are due to three 

 causes ; abortion, degeneration, and adherence (adherence). 

 These three causes, by which the original symmetry of a class 

 is changed and may even be utterly obscured, are then fully 

 illustrated by examples. 



In respect to abortion he distinguishes that which is pro- 

 duced by internal causes from that which is due to accidental 

 and external ones ; he refers especially to the abortion of two 

 loculaments in the fruit of the horse-chestnut and the oak, to 

 the suppression of the terminal bud in some shrubs by the 

 adjoining axillary buds, and to the fact that all organs of 

 plants may become abortive in a similar manner ; for instance, 

 the sexual organs disappear entirely in the disk-flowers of 

 Viburnum Opulus, and one of the two sexes in the flower 

 of Lychnis dioica. He goes on to answer the question, how 

 it is possible to discover the symmetry in such cases ; one 

 method he finds supplied by monstrosities, among which 

 there are even some that may be regarded as a return to 

 the original symmetry, the cases known as peloria. Analogy 



