378 SUMMARY. [CHAP. XIV. 



still retained in the spelling, but become useless in the pronuncia- 

 tion, 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 difficulty, as they assuredly 

 do on the old doctrine of creation, might even have been antici- 

 pated 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 circuitous 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 characters, 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 extinc- 

 tion 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 ranking together the sexes, 

 ages, dimorphic forms, and acknowledged varieties of the same 

 species, however much they may differ from each other in struc- 

 ture. 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 genea- 

 logical in its attempted arrangement, with the grades of acquired 

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

 orders, and classes. 



On this same view of descent with modification, most of the 

 great facts in Morphology become intelligible, whether we look 

 to the same pattern displayed by the different species of the same 

 class in their homologous organs, to whatever purpose applied ; or 

 to the serial and lateral homologies in each individual animal and 

 plant. 



On the principle of successive slight variations, not necessarily 

 or generally supervening at a very early period of life, and being 

 inherited at a corresponding period, we can understand the leading 

 facts in Embryology ; namely, the close resemblance in the indi- 

 vidual embryo of the parts which are homologous, and which 

 when matured become widely different in structure and function ; 

 and the resemblance of the homologous parts or organs in allied 



