426 THE ORIGIN OF SPECIES 



slowly acquired through natural selection, we need not marvel at 

 some instincts being not perfect and liable to mistakes, and at 

 many instincts causing other animals to suffer. 



If species be only well-marked and permanent varieties, we 

 can at once see why their crossed offspring should follow the same 

 complex laws in their degrees and kinds of resemblance to their 

 parents — in being absorbed into each other by successive crosses, 

 and in other such points — as do the crossed offspring of acknowl- 

 edged varieties. This similarity would be a strange fact, if species 

 had been independently created, and varieties had been produced 

 through secondary laws. 



If we admit that the geological record is imperfect to an ex- 

 treme degree, then the facts, which the record does give, strongly 

 support the theory of descent with modification. New species have 

 come on the stage slowly and at successive intervals; and the 

 amount of change, after equal intervals of time, is widely different 

 in different groups. The extinction of species and of whole groups 

 of species, which has played so conspicuous a part in the history 

 of the organic world, almost inevitably follows from the prin- 

 ciple of natural selection; for old forms are supplanted by new 

 and improved forms. Neither single species nor groups of species 

 reappear when the chain of ordinary generation is once broken. 

 The gradual diffusion of dominant forms, with the slow modifica- 

 tion of their descendants, causes the forms of life, after long in- 

 tervals of time, to appear as if they had changed simultaneously 

 throughout the world. The fact of the fossil remains of each forma- 

 tion being in some degree intermediate in character between the 

 fossils in the formations above and below, is simply explained by 

 their intermediate position in the chain of descent. The grand fact 

 that all extinct beings can be classed with all recent beings, natu- 

 rally follows from the living and the extinct being the offspring 

 of common parents. As species have generally diverged in char- 

 acter during their long course of descents and jnodification , we 

 can understand why it is that the more ancient forms, or early 

 progenitors of each group, so often occupy a position in some 

 degree intermediate between existing groups. Recent forms are 

 generally looked upon as being, on the whole, higher in the scale 

 of organization than ancient forms; and they must be higher, in 

 so far as the later and more improved forms have conquered the 

 older and less improved forms in the struggle for life; they have 

 also generally had their organs more specialized for different func- 

 tions. This fact is perfectly compatible with numerous beings still 

 retaining simple and but little improved structures, fitted for 



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