274 THE ORIGIN OF SPECIES 



as with rudimentary organs, of no importance — the wide 

 opposition in value between analogical or adaptive char- 

 acters, and characters of true affinity; and other such 

 rules; — all naturally follow if we admit the common 

 parentage of allied forms, together with their modifica- 

 tion 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 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 

 aiTangement, 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 correspond- 

 ing 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 



