Chap. XIV. EECAPITULATION. 475 
yet should follow nearly the same instincts; why the 
thrush of South America, for instance, lines her nest 
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 
apparently not perfect and liable to mistakes, and at 
many instincts causing other animals to suffer. 
If species be only well-marked and permanent varie¬ 
ties, 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 ab¬ 
sorbed into each other by successive crosses, and in other 
such points,—as do the crossed offspring of acknow¬ 
ledged varieties. On the other hand, these would be 
strange facts if species have been independently created, 
and varieties have been produced by secondary laws. 
If we admit that the geological record is imperfect 
in an extreme degree, then such facts as the record 
gives, 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 inevitabljT^ follows 
on the principle of natural selection; for old forms vvill 
be supplanted by new and improved forms. Neither 
single species nor groups of species reappear when the 
chain of ordinary generation has once been 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 
