SUMMARY 497 



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 be- 

 tween 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 selec- 

 tion, 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, dimor- 

 phic 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 neces- 

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

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

 stand the 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 of the 

 homologous parts or organs in allied though distinct species, 

 though fitted in the adult state for habits as different as is 

 possible. Larvc-e are active embryos, which have been spe- 

 cially modified in a greater or less degree in relation to their 

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

 sponding early age. On these same principles, — and bearing 

 in mind that when organs are reduced in size, either from 



