Chap. XV. Recapitulation. 417 



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 acknow- 

 ledged 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 extreme 

 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 principle of 

 natural selection; for old forms are supplanted by new and im- 

 proved forms. Neither single species nor groups of species re- 

 appear when the chain of ordinary generation is once broken. The 

 gradual diffusion of dominant forms, with the slow modification of 

 their descendants, causes the forms of life, after long intervals 

 of time, to appear as if they had changed simultaneously through- 

 out the world. The fact of the fossil remains of each formation 

 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, 

 naturally follows from the living and the extinct being the off- 

 spring of common parents. As species have generally diverged in 

 character during their Ions; course of descent and modification, 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 organisation than ancient forms ; and they must be higher, in 

 s o 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 specialised for different 

 functions. This fact is perfectly compatible with numerous beings 

 still retaining simple and but little improved structures, fitted for 

 simple conditions of life; it is likewise compatible with some 



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