Page:Popular Science Monthly Volume 8.djvu/442

426 insectivorous or carnivorous, and their teeth vary much in number and structure. Certain of my readers may wonder that such diverse forms should be thus grouped together, apart from the other mammals. At first sight it might seem more natural to place together flying opossums with flying squirrels; the native sloth with the true sloth; the dog and cat-like opossums with the true dogs and cats; and, lastly, the insectivorous marsupials with the other insectivora.

As to the kangaroos themselves, they might be considered as approximating in one respect to the Ruminants, in another to the Rodents.

We have seen that even in Captain Cook's time its resemblance to the jerboa forced itself into notice. And, indeed, in this jerboa (and its first cousin, the alactaga) we have the same or even a relatively greater length of hind-limb and tail, and we have the same jumping mode of progression.

Again, in the little jumping insectivorous mammal, the shrew (Macroscelides), we meet with excessively long hind-limbs and a jumping habit. More than this: if we examine its teeth, we find both in the upper cutting teeth and in the back teeth great resemblance to those of the kangaroo. And yet there is no real affinity between the kangaroo and such creatures, any more than there is between a non-marsupial truly carnivorous beast and a marsupial carnivore. Indeed, both myself and my readers are far more like the jerboa or weasel than either of the latter is like to any marsupial animal.

The fact is, that all these so varied marsupial forms of life possess in common certain highly-important characters, by which they differ from all other mammals. These characters, however, mainly relate to the structure of their reproductive organs, and could not be here detailed without a long preliminary anatomical explanation; but, as to the great importance of these characters, naturalists are agreed.

Among the characters which serve to distinguish the marsupials, there are two to which I have already called attention in describing the kangaroo; namely, the marsupial bones and the inflected angle of the lower jaw.

Every mammal which has marsupial bones has the angle of its jaw inflected, or else has no angle to its jaw at all; while every animal which has both marsupial bones and an inflected jaw-angle possesses also those special characters of the reproductive system which distinguish the marsupials from all other mammals.

Thus it is clear we have at least two great groups of mammals. One of them—the non-marsupials—contains man; the apes; bats; hedgehog-like beasts (shrews, moles, etc.); cats, dogs, bears, etc.; hoofed beasts; edentates; rodents, and also the aquatic mammals. And this great group, containing so many orders, is named