426 TTTJ^ POPULAR SCIENCE MONTHLY. 



sively insectivorous or carnivorous, and their teeth vary much in num- 

 ber and structure. Certain of my readers may wonder that such di- 

 verse forms should be thus grouped together, apart from the other 

 mammals. At first sight it might seem more natural to place togeth- 

 er flying opossums with flying squirrels y the ?iaHve sloth with the 

 true sloth ; tlie dog and cat-like ojjossiwis with the true dogs and cats j 

 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 tlje same jumping 

 mode of progression. 



Again, iji the little jumping insectivorous mammal, the shrew 

 (Ifaeroscelides), 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 resem- 

 blance to those of the kangaroo. And yet there is no real afliuity be- 

 tween the kangaroo and such creatures, any more than thei'e is be- 

 tween a non-marsupial truly carnivorous beast and a marsupial car- 

 nivore. Indeed, both myself and ray 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 difi:er 

 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 ani- 

 mal which has both marsupial bones and an inflected jaw-angle pos- 

 sesses 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-mai'supials 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 Mono- 



DELPHIA. 



