204 
DIFFICULTIES ON THEOEY. 
Chap. VI. 
tlie two forms whicli it connects ; consequently the two 
latter, during the course of further modification, from 
existing in greater numbers, will have a great advantage 
over the less numerous intermediate variety, and will 
thus generally succeed in supplanting and extermi¬ 
nating it. 
We have seen in this chapter how cautious we should 
be in concluding that the most different habits of life 
could not graduate into each other; that a bat, for 
instance, could not have been formed by natural selec¬ 
tion from an animal which at first could only glide 
through the air. 
We have seen that a species may under new condi¬ 
tions of life change its habits, or have diversified habits, 
with some habits very unlike those of its nearest con¬ 
geners. Hence we can understand, bearing in mind 
that each organic being is trying to live wherever it 
can live, how it has arisen that there are upland geese 
with webbed feet, ground woodpeckers, diving thrushes, 
and petrels with the habits of auks. 
Although the belief that an organ so perfect as the 
eye could have been formed by natural selection, is 
more than enough to stagger any one ; yet in the case 
of any organ, if we know of a long series of gradations 
in complexity, each good for its possessor, then, under 
changing conditions of life there is no logical impossi¬ 
bility in the acquirement of any conceivable degree of 
perfection through natural selection. In the cases in 
which we know of no intermediate or transitional states, 
we should be very cautious in concluding that none 
could have existed, for the homologies of many organs 
and their intermediate states show that wonderful meta¬ 
morphoses in function are at least possible. For instance, 
a swim-bladder has apparently been converted into an 
air-breathing lung. The same organ having performed 
