Chap. XV.] RECAPITULATION. 419 



cross wide spaces of the ocean, as frogs and terrestrial mammals, do 

 not inhabit oceanic islands ; and why, on the other hand, new and 

 peculiar species of bats, animals which can traverse the ocean, aro 

 often found on islands far distant from any continent. Such cases 

 as the presence of peculiar species of bats on oceanic islands and 

 the absence of all other terrestrial mammals, are facts utterly 

 inexplicable on the theory of independent acts of creation. 



The existence of closely allied or representative species in any 

 two areas, implies, on the theory of descent with modification, that 

 the same parent-forms formerly inhabited both areas : and we 

 almost invariably find that wherever many closely allied species 

 inhabit two areas, some identical species are still common to both. 

 Wherever many closely allied yet distinct species occur, doubtful 

 forms and varieties belonging to the same groups likewise occur, 

 It is a rule of high generality that the inhabitants of each area are 

 related to the inhabitants of the nearest source whence immigrants 

 might have been derived. We see this in the striking relation ot 

 nearly all the plants and animals of the Galapagos archipelago. 

 of Juan Fernandez, and of the other American islands, to the 

 plants and animals of the neighbouring American mainland ; and 

 of those of the Cape de Verde archipelago, and of the other African 

 islands to the African mainland. It must be admitted that these 

 facts receive no explanation on the theory of creation. 



The fact, as we have seen, that all past and present organic beiugs 

 can be arranged within a few great classes, in groups subordinate to 

 groups, and with the extinct groups often falling in between the 

 recent groups, is intelligible on the theory of natural selection with 

 its contingencies of extinction and divergence of character. On 

 these same principles we see how it is, that the mutual affinities of 

 the forms within each class are so complex and circuitous. We 

 see why certain characters are far more serviceable than others for 

 classification ; — why adaptive characters, though of paramount im- 

 portance to the beings, are of hardlv any importance in classifi- 

 cation ; why characters derived from rudimentary parts, though of 

 no service to the beings, are often of high classificatory value ; and 

 why embryological characters are often the most valuable of alL 

 The real affinities of all organic beings, in contradistinction to their 

 adaptive resemblances, are due to inheritance or community of 

 descent. The Natural System is a genealogical arrangement, with 

 the acquired grades of difference, marked by the terms, varieties, 

 species, genera, families, &c. ; and we have to discover the lines 

 cf descent by the most permanent characters whatever they may 

 be and of however slight vital importance. 



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