4i}'4 iiVMMABt. 



proper places in the natural system, have often found rudi- 

 mentary parts as useful as, or even sometimes more useful 

 than, parts of high physiological importance. Eudimentary 

 organs may be compared with the letters in a word, still 

 retained in the spelling, but become useless in the pro- 

 nunciation, but which serve as a clue for its derivation. 

 On the view of descent with modification, we may conclude 

 that the existence of organs in a rudimentary, imperfect, 

 and useless condition, or quite aborted, far from presenting 

 a strange diflBculty, as they assuredly do on the old doctrine 

 of creation, might even have been anticipated in accordance 

 with the views here explained. 



SUMMARY. 



In this chapter I have attempted to show that the 

 arrangement of all organic beings throughout all time in 

 groups under groups — that the nature of the relationships 

 by which all living and extinct organisms are united by 

 complex, radiating, and circuituous lines of affinities into 

 a few grand classes — the rules followed and the difficulties 

 encountered by naturalists in their classifications — the 

 value set upon characters, if constant and prevalent, 

 whether of high or of the most trifling importance, or, as 

 with rudimentary organs of no importance — the wide 

 opposition in value between analogical or adaptive charac- 

 ters, and characters of true affinity; and other such rules; 

 ■ — all naturally follow if we admit the common parentage 

 of allied forms, together with their modification through 

 variation and natural selection, with the contingencies of 

 extinction and divergence of character. In considering 

 this view of classification, it should be borne in mind that 

 the element of descent has been universally used in rank- 

 ing together the sexes, ages, dimorphic forms, and acknowl- 

 edged varieties of the same species, however much they 

 may differ from each other in structure If we extend the 

 use of this element of descent — the one certainly known 

 cause of similarity in organic beings — we shall understand 

 what is meant by the Natural System: it is genealogical 

 in its attempted arrangement, with the grades of acquired 

 difference marked by the terms, varieties, species, genera, 

 families, orders, and classes. 



