to ordinary
birds, but to the struthious ones from certain extinct reptiles termed
Dinosauria; one of the most familiarly known of which is the Iguanodon of
the Wealden formation. In these Dinosauria we find skeletal characters
unlike those of ordinary (_i.e._ carinate) birds, but closely resembling in
certain points the osseous structure of the struthious birds. Thus a
difficulty presents itself as to the explanation of the three following
relationships:--(1) That of the Pterodactyles with carinate birds; (2) that
of the Dinosauria with struthious birds; (3) that of the carinate and
struthious birds with each other.

Either birds must have had two distinct origins whence they grew to their
present conformity, or the very same skeletal, and probably cerebral
characters must have spontaneously and independently arisen. Here is a
dilemma, either horn of which bears a threatening aspect to the exclusive
supporter of "Natural Selection," and between which it seems somewhat {72}
difficult to choose.

It has been suggested to me that this difficulty may be evaded by
considering pterodactyles and carinate birds as independent branches from
one side of an ancient common trunk, while similarly the Dinosauria and
struthious birds are taken to be independent branches from the other side
of the same common trunk; the two kinds of birds resembling each other so
much on account of their later development from that trunk as compared with
the development of the reptilian forms. But to this it may be replied that
the ancient common stock could not have had at one and the same time a
shoulder structure of _both kinds_. It must have been that of the
struthious birds or that of the carinate birds, or something different from
both. If it was that of the struthious birds, how did the pterodactyles and
carinate birds independently arrive at the very same divergent structure?
If it was that of the carinate birds, how did the struthious birds and
Dinosauria independently agree to differ? Finally, if it was something
different from either, how did the carinate birds and pterodactyles take on
independently one special common structure when disagreeing in so many;
while the struthious birds, agreeing in many points with the Dinosauria,
agree yet more with the carinate birds? Indeed by no arrangement of
branches from a stem can the difficulty be evaded.

Professor Huxley seems inclined[55] to cut the Gordian knot by considering
the shoulder str