CHAPTER III.
THE CO-EXISTENCE OF CLOSELY SIMILAR STRUCTURES OF DIVERSE ORIGIN.
Chances against concordant variations .-- Examples of discordant ones .-- Concordant variations not unlikely on a non-Darwinian evolutionary hypothesis .-- Placental and implacental mammals .-- Birds and reptiles .-- Independent origins of similar sense organs .-- The ear .-- The eye .-- Other coincidences .-- Causes besides Natural Selection produce concordant variations in certain geographical regions .-- Causes besides Natural Selection produce concordant variations in certain zoological and botanical groups .-- There are homologous parts not genetically related .-- Harmony in respect of the organic and inorganic worlds .-- Summary and conclusion.
The theory of "Natural Selection" supposes that the varied forms and structure of animals and plants have been built up merely by indefinite, fortuitous,[49] minute variations in every part and in all directions--those variations only being preserved which are directly or indirectly useful to the individual possessing them, or necessarily correlated with such useful variations.
[Illustration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.
(_Copied, by permission, from Mr.Andrew Murray's "Geographical Distribution of Mammals."_)] On this theory the chances are almost infinitely great against the independent, accidental occurrence and preservation of two similar series of minute variations resulting in the independent development of two closely similar forms.

In all cases, no doubt (on this same theory), _some_ adaptation to habit or need would gradually be evolved, but that {64} adaptation would surely be arrived at by different roads.

The organic world supplies us with multitudes of examples of similar functional results being attained by the most diverse means.

Thus the body is sustained in the air by birds and by bats.

In the first case it is so sustained by a limb in which the bones of the hand are excessively reduced, but which is provided with immense outgrowths from the skin--namely, the feathers of the wing.