54 TYPES OF ANIMAL LIFE 



The other hypothesis is that the whole of one group 

 descended from one small section of the other, either all 

 placentals from some one marsupial species, or all mar- 

 supials from one non-marsupial form. It is the latter 

 hypothesis which is now in vogue, and the favourite 

 opinion at present is that all marsupials descended from 

 some insectivorous beast not very unlike a hedgehog, 

 minus his spikes. But on this hypothesis, again, it is 

 absolutely necessary that a number of very similar 

 structures must be affirmed to have arisen independently. 

 Such, in fact, must have been the case with all those 

 structural and functional characters by which the various 

 groups of marsupials, resemble the various parallel groups 

 of placental beast.^!. Thus it is that the opossum and its 

 allies exemplify " the independent origin of similar 

 structures " more convincingly than almost any other 

 order of beasts. They do so indeed in a way which 

 makes denial simply impossible. 



But among themselves alone, they force on our obser- 

 vation a subordinate instance of the same thing, and this 

 is why we call attention to the various forms of structure 

 presented by the marsupial hind-foot. We found its 

 second and third toes becoming more and more bound 

 together as we passed from the wombat, through the 

 phalangers and bandicoots, to the kangaroos; while in 

 the carnivorous forms, as also in the American opossum, 

 these toes are as well developed and as independent as 

 the others. We wUl call the former set, Group A., and 

 the latter set. Group B. One of two alternatives, then, 

 we must admit : either the forms contained in each group 

 are specially connected by blood relationship and descent or 

 they are not. If they are, then the various resemblances 

 which may be detected between them, and which cannot 

 be thought due to descent from a common ancestor, are 



