MUTUAL AFFINITIES OF ORGANIC BEINGS 411 



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 com- 

 mon 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 de- 

 scent 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 

 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 

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

 as different as is possible. Larvae are active embryos, which have 



