THE NATURAL HISTORY OF THE KANGAROO. 
389 
and their teeth vary much in number and structure. Cer- 
tain 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 resem- 
blance 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 are 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 prelimi- 
nary anatomical explanation ; but as to the great importance of 
these characters, naturalists are agreed. 
Amongst the characters which serve to distinguish the mar- 
supials, 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 repro- 
ductive system which distinguish the marsupials from all other 
mammals. 
Thus it is clear we have at least two great groups of mam- 
mals. One of them — the non-marsupials — contains Man ; the 
Apes; Bats; Hedgehog-like Beasts (Shrews, Moles, &c.); Cats, 
Dogs, Bears, &c. ; Hoofed Beasts ; Edentates ; Rodents, and also 
