410 SUMMARY. Cuap. XIII. 



as they assuredly do on the ordinary doctrine of creation^ 

 might even have been anticipated in accordance with the ^'iews 

 here explained. 



Summary. 



In this chapter I have attempted to show that the arrange- 

 ment of all organic beings throughout all time in group under 

 group — that the nature of the relationship, by which all living 

 and extinct organisms are united by complex, radiating, and 

 circuitous lines of affinities in 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 the most trifling importance, or, 

 as with rudimentary organs, of no importance — the wide oppo- 

 sition in value between analogical or adaptive characters, and 

 characters of true aflinity, and other such rules — all naturally 

 follow if we admit the common parentage of allied forms, to- 

 gether with their modification through natural selection, with 

 its contingencies of extinction and divergence of character. 

 In considering tliis view of classification, it should be borne in 

 mind that the element of descent has been universally used in 

 ranlcing together the sexes, ages, dimoi-jihic forms, and acknowl- 

 edged varieties of the same species, however different they 

 may be in structure. If we extend the use of this element of 

 descent — the one certainly known cause of similarity in or- 

 ganic beings — we shall understand what is meant by the Nat- 

 ural System : it is genealogical in its attempted arrangement, 

 with the grades of acquired difference marked by the terms, 

 varieties, species, genera, families, orders, arid classes. 



On this same view of descent with modification, all 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 pur- 

 pose applied ; or to the homologous parts in each individual 

 animal and plant. 



On this principle of successive slight variations, not neces- 

 sarily or generally supervening at a very early period of life, 

 and being inherited at a corresponding period, we can under- 

 stand the great leading facts in Embryology ; namely, the 

 close resemblance in the individual embryo of the parts which 

 are homologous, and which when matured become widely dif- 

 ferent in structure and function ; and the resemblance in allied 

 though very distinct species of their homologous parts or 



