rged that even very complex extremely similar
structures have again and again been developed quite independently one of
the other, and this because the process has taken place not by merely
haphazard, indefinite variations in all directions, but by the concurrence
of some other and internal natural law or laws co-operating with external
influences and with Natural Selection in the evolution of organic forms.

It must never be forgotten that to admit any such constant operation of any
such unknown natural cause is to deny the purely Darwinian theory, which
relies upon the survival of the fittest by means of minute fortuitous
indefinite variations.

Amongst many other obligations which the Author has to acknowledge to
Professor Huxley, are the pointing out of this very difficulty, and the
calling his attention to the striking resemblance between certain teeth of
the dog and of the thylacine as one instance, and certain ornithic
peculiarities of pterodactyles as another.

Mammals[51] are divisible into one great group, which comprises the {68}
immense majority of kinds termed, from their mode of reproduction,
_placental Mammals_, and into another very much smaller group comprising
the pouched-beasts or marsupials (which are the kangaroos, bandicoots,
phalangers, &c., of Australia), and the true opossums of America, called
_implacental Mammals_. Now the placental mammals are subdivided into
various orders, amongst which are the flesh-eaters (Carnivora, _i.e._ cats,
dogs, otters, weasels, &c.), and the insect-eaters (Insectivora, _i.e._
moles, hedgehogs, shrew-mice, &c.). The marsupial mammals also present a
variety of forms (some of which are carnivorous beasts, whilst others are
insectivorous), so marked that it has been even proposed to divide them
into orders parallel to the orders of placental beasts.

The resemblance, indeed, is so striking as, on Darwinian principles, to
suggest the probability of genetic affinity; and it even led Professor
Huxley, in his Hunterian Lectures, in 1866, to promulgate the notion that a
vast and widely-diffused marsupial fauna may have existed anteriorly to the
development of the ordinary placental, non-pouched beasts, and that the
carnivorous, insectivorous, and herbivorous placentals may have
respectively descended from the carnivorous, insectivorous, and herbivorous
marsupials.

[Illustration: TEETH OF UROTRICHUS AND PERAMELES.]

Amongst other points Professor Huxley called atte