INTRODUCTION 59 



grow on heaths are an expression of the adaptation 

 of the different plants that inhabit them. Not all the 

 floristic elements of a heath are of the same order, 

 however, nor are the different organs of each diverse 

 plant-form found on a heath of the same type, but in 

 the main the type of adaptation in each case is more 

 or less similar in the nature of the work the particu- 

 lar organ is called upon to perform. The diversity 

 of the plant types thus evokes a difference of response 

 according as the ancestral type of each varies from 

 the rest of the components of a heath formation. 



By comparing the forms of the same organs in 

 allied plants, or by comparative anatomy, or mor- 

 phology, one may determine the probable history of 

 such forms, or how one form has been derived from 

 another, or the sequence of such changes. Similar 

 organs are homologous, and are more easily distin- 

 guished in the more recent forms than in the earlier 

 ones. 



In the case of early ancestral forms there are 

 generalised types which combine the characters 

 assumed by several later types, and from which the 

 latter may be derived. 



One set of organs may be modified, as leaves into 

 spines, etc. These facts also illustrate the sequence of 

 the evolution of one set of organs from another, or 

 the development of each form of organ. 



Thus the floral leaves may be regarded as perhaps 

 derived from the foliage leaves. The history of the 

 individual, or the study of ontology, helps to elucidate 



