514 ORIGIN OF SPECIES 



neuter insects, which leave no progeny to inherit the effects 

 of long-continued habit. On the view of all the species of 

 the same genus having descended from a common parent, 

 and having inherited much in common, we can understand 

 how it is that allied species, when placed under widely dif- 

 ferent conditions of life, yet follow nearly the same in- 

 stincts; why the thrushes of tropical and temperate South 

 America, for instance, line their nests with mud like our 

 British species. On the view of instincts having been 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 resem- 

 blance to their parents, — in being absorbed into each other 

 by successive crosses, and in other such points, — as do the 

 ' crossed oft'spring of acknowledged 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 succes- 

 sive intervals; and the amount of change, after equal inter- 

 vals 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 nat- 

 ural 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 modification of their descendants, causes the forms 

 of life, after long intervals of time, to appear as if they had 

 changed simultaneously throughout the world. The fact of 

 the fossil remains of each formation being in some degree 

 intermediate in character between the fossils in the forma- 

 tions above and below, is simply explained by their inter= 



