PRINCIPLES OF CLASSIFICATION. 



57 



the fact that the existence of such a gradational series entirely does away with the fundamental 

 assumption on which tlie idea of a natural species (as ordinarily understood) is based. When 

 a large collection of individuals of one generic type is brought together (such as that placed 

 in my hands by Mr. Gumming of the Opcrmlina of the Philippine Seas), it very commonly 

 happens that by selecting the most divergent forms, a considerable number of specific types — 

 say six, eight, twelve, or even twenty — might be readily establislicd, and a considerable part of 

 the collection might be arranged around these as centres ; but after all the specimens have 

 being thus separated, which present a sufSciently close conformity to those types to admit of 

 been referred to one or other of them without hesitation, there will remain a considerable 

 proportion in which the characters of two or more are combined with such equality as to 

 render it impossible to assign to them any other than an intermediate position, whilst there 

 will be others which present such departures from any of them as themselves to have an 

 equal claim to rank as distinct species ; so that there is no middle course between that 

 of grouping the whole series as varieties of one species, and that of erecting into a distinct 

 species every varietal modification presented by individuals, — a course which would be the 

 ■reductio ad absurdum of the ordinary system of species-making in its application to this group. 



71. Two sets of characters may be especially named, on which it has been customary to 

 found specific distinctions ; these are the form of the septal plane, and external sculpture or 

 stirface-markini/. Now, in regard to the first of these, it may be affirmed most positively 

 that wherever any marked variation exists, that variation will be found, on comparison of a 

 sufficient number of specimens, to be so gradational as to defy all attempts to use it as a 

 basis of specific differentiation. Of this we have a marked example in Operculina, six vertical 

 sections of which are shown in Fig. IX. Further, no dissimilarity between the form of the 



Fig. IX. 



e.. 



Vertical sectious of six specimeus of Operculina ; showing a remarkable diversity in tlie forms and proportions of the chambers 



8 



