vin ESSENTIAL AND ADAPTIVE CHARACTERS 117 



to one great branch of the mammalian class, although they 

 represent different stocks of that branch. The thylacine 

 belongs to another primary branch, but in that branch it has 

 taken the place, functionally, of the dog in the other; its 

 organs have become adapted to perform the same work. 1 



It has been asserted, in a recent very able pamphlet by an 

 anonymous writer, " On the Difficulties of the Theory of 

 Natural Selection," 2 " that on the theory of ' natural selection ' 

 it is all but impossible, such are the probabilities against it, 

 that identical structures should have arisen independently. 

 Yet many structures undeniably exist which, to all appearance, 

 must have so arisen." And the " remarkable identity of 

 structure between certain of the teeth of the large predatory 

 marsupial, the thylacine, or Tasmanian wolf, and those of the 

 common dog," is cited as a case in point. Now, though I am 

 fully willing to admit difficulties in the theory, quite 

 inexplicable with our present knowledge, as many almost as 

 the author quoted, I cannot see this one in the same light. It 

 appears to me that the probabilities, instead of being against 

 the independent origin of such similar structures (erroneously 

 called identical) as those just mentioned, are exceedingly in 

 their favour. I cannot in the least see why a marsupial 

 animal should not be carnivorous and predaceous as well as a 

 placental animal, and if so, why its teeth should not have 

 come to possess the general attributes essential to animals of 

 such habits namely, large, pointed, recurved fangs at each of 

 the anterior angles^ of the jaw ; the teeth between them much 

 reduced, so as not to interfere with the piercing and holding 

 action of the canines ; the molars more or less scissor-shaped. 



1 The two great branches of the mammalia referred to are (1) the 

 Monodelphous, or placental, comprising the great bulk of the animals of the 

 class ; and (2) the Didelphous, or nonplacental. The latter, which are the 

 animals commonly known as Marsupialia, or pouched animals, though varying 

 extremely in the structure of the feet, teeth, etc., also in their food and manner 

 of life, all agree together, and differ from the monodelphia in numerous 

 important characters of the skeleton, brain, heart, and especially of the 

 reproductive organs and processes. The marsupials are, at the present day, 

 entirely confined to Australia, its neighbouring islands, and the American 

 continent, though, in former times, they had a more extensive geographical 

 range. 



2 Published originally in the Month, 1869. 



