m of a sac, which
changes into the aqueous humour and lens. An outgrowth of brain substance,
on the other hand, forms the retina, while a third process is a lateral
ingrowth of connective tissue, which afterwards changes into the vitreous
humour of the eye.

The internal ear is formed by an involution of the integument, and not by
an outgrowth of the brain. But tissue, in connexion with it, becomes in
part changed, thus forming the auditory nerve, which places the tegumentary
sac in direct communication with the brain itself.

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Now, these complex and simultaneous co-ordinations could never have been
produced by infinitesimal beginnings, since, until so far developed as to
effect the requisite junctions, they are useless. But the eye and ear when
fully developed present conditions which are hopelessly difficult to
reconcile with the mere action of "Natural Selection." The difficulties
with regard to the eye have been well put by Mr. Murphy, especially that of
the concordant result of visual development springing from different
starting-points and continued on by independent roads.

He says,[41] speaking of the beautiful structure of the perfect eye, "The
higher the organization, whether of an entire organism or of a single
organ, the greater is the number of the parts that co-operate, and the more
perfect is their co-operation; and consequently, the more necessity there
is for corresponding variations to take place in all the co-operating parts
at once, and the more useless will be any variation whatever unless it is
accompanied by corresponding variations in the co-operating parts; while it
is obvious that the greater the number of variations which are needed in
order to effect an improvement, the less will be the probability of their
all occurring at once. It is no reply to this to say, what is no doubt
abstractedly true, that whatever is possible becomes probable, if only time
enough be allowed. There are improbabilities so great that the common sense
of mankind treats them as impossibilities. It is not, for instance, in the
strictest sense of the word, impossible that a poem and a mathematical
proposition should be obtained by the process of shaking letters out of a
box; but it is improbable to a degree that cannot be distinguished from
impossibility; and the improbability of obtaining an improvement in an
organ by means of several spontaneou