In the spider, we have a specially constructed antenna; and finally in the centipede a pair of modified thoracic limbs.
[Illustration: A CENTIPEDE.] It would be easy to produce a multitude of such instances of similar ends being attained by dissimilar means, and it is here contended that by "the action of Natural Selection" _only_ it is so improbable as to be practically impossible for two exactly similar structures to have ever been independently developed.

It is so because the number of possible {67} variations is indefinitely great, and it is therefore an indefinitely great number to one against a similar series of variations occurring and being similarly preserved in any two independent instances.
The difficulty here asserted applies, however, only to pure Darwinism, which makes use _only_ of indirect modifications through the survival of the fittest.
Other theories (for example, that of Mr.Herbert Spencer) admit the _direct_ action of conditions upon animals and plants--in ways not yet fully understood--there being conceived to be at the same time a certain peculiar but limited power of response and adaptation in each animal and plant so acted on.

Such theories have not to contend against the difficulty proposed, and it is here urged 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 attention to the resemblance between the anterior molars of the placental dog with those of the marsupial thylacine.