Chap. XIV. Summary. 403 



and divergence of character. In considering this view of classifica- 

 tion, 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 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 arrange- 

 ment, 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 though 

 distinct species, though fitted in the adult state for habits as 

 different as is possible. Larva? are active embryos, which have 

 been specially modified in a greater or less degree in relation to their 

 habits of life, with their modifications inherited at a correspond- 

 ing early age. On these same principles, — and bearing in mind, 

 that when organs are reduced in size, either from disuse or through 

 natural selection, it will generally be at that period of life when 

 the being has to provide for its own wants, and bearing in mind 

 how strong is the force of inheritance — the occurrence of rudi- 

 mentary organs might even have been anticipated. The im- 

 portance of embryological characters and of rudimentary organs 

 in classification is intelligible, on the view that a natural arrange- 

 ment must be genealogical. 



Finally, the several classes of facts which have been considered 

 in this chapter, seem to me to proclaim so plainly, that the innu- 

 merable species, genera and families, with which this world is 

 peopled, are all descended, each within its own class or group, from 

 common parents, and have all been modified in the course of descent, 

 that I should without hesitation adopt this view, even if it were 



unsupported by other facts or arguments. 



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