CLASSIFICATION. 53 



This is not worth much, the record being so very imperfect, from the greater part of the 

 earth being now under water, or haying been in ancient times above water, and hence having 

 furnished few or no organic remains. But such as it is, it gives us the Marsupials (with a 

 questionable trace of Insectivora) as the first Mammals which appeared on the face of the earth. In 

 addition to the geological reasons for believing in their early appearance, the fact of their structure 

 being lowest in the scale of Mammalian life is an important fact, for although there is no doubt that 

 steps are taken by nature both backwards and forwards, as well as to the right and to the left, stiU 

 her general course iias been forwards, — her motto has always been "Excelsior," — and the chances 

 are thus in favour of the lowest having appeared first. The admirable system of classification 

 (admirable from the point of view of organization) founded by Professor Owen, chiefly, although 

 by no means wholly, on the characters of the brain,* seems to place it beyond doubt that the 

 relative position of the Marsujiials is at the bottom of the list, and that the place of the Rodents 

 and Insectivora is next to them. At any rate that one or other of these three is oldest. If 

 the Marsupials are not the oldest, then the choice lies between the Insectivora and the Rodents, 

 the claim of the former being strongest on geological grounds, of the latter on structural, and both 

 (if we reckon the Bats as Insectivores) as being the only placental inhabitants found in Australia 

 along with the Marsupials ; the equivalent relations above referred to being also much stronger 

 with them than with any other order. 



These equivalent groups are of two kinds. One disj^lays relations of analogy which have been 

 thought to tj'pify difierent orders of placentals ; as, for example, the pedimanous and frugivorous 

 opossums are supposed to have foreshadowed the pedimanous frugivorous monkeys ; the marsupial 

 hyena or tiger (Thylacixus) our common carnivora ; the wombat, the rodents ; the kangaroo, the 

 ruminants ; the koala, the phytophagous sun-bear ; the phascogales, the shrews ; the Ecliidna and 

 Myrmecobius, the ant-eater. This relation shows parallelism of internal structure, but little external 

 resemblance. The cow has a complicated digestive apparatus ; so has the kangaroo. Both are her- 

 bivorous ; herbivorous animals require a more elaborate apf)aratus for digestion than carnivorous (which 

 have a great part of the business of assimilation from vegetable to animal tissues already done to 

 their hand), therefore both are supplied ^ith suitable machinery for the purpose; it is not the 

 same, but so far alike as is necessary. So with their dentition ; in both it is adapted for grinding 

 vegetable matter, but it is not the same. As to external appearance, on the other hand, they are 

 totally void of resemblance. This kind of analogy is more of the nature of homology than of close 

 affinity. 



The other analogy is of a difierent character. It is a close personal resemblance promising abso- 

 lute identity throughout, but not fulfilling the promise. The frontispiece is an illustration of this sort 

 of resemblance. It is a representation of Antechinus minutissimus and Mus delicatxjlus, 

 two Australian Mammals, copied, by Mr. Gould's permission, from figures in his magnificent 

 work on the Mammals of Australia. The one belongs to the order of marsupials, the other does 

 not ; the one has rodent dentition, the other has not ; and yet, as the reader sees, the two are so 

 identical in outward appearance, that, on a hasty inspection the most experienced naturalist might 

 be deceived, and might set them down as two species of mice. The same thing happens with 

 other species of rodents ; — the flying Marsupial Petaurus is a close counterpart, in outward 

 appearance, of some of the flying squirrels. One or two of the Phascogales, or Antechini, resemble 



* Owen, in " Proceedings of Linnean Society," vol. ii. p. i. 1857. 



