52 



MAMMALS. 



a group from which another has sprung may still subsist in its descendants; some groups may- 

 be very old, others quite modern, alwaj^s supposing that the groups really do exist in nature, and 

 are homogeneous, and may be treated as entities. If we may so treat them, the best substitute 

 for a linear arrangement is to have as few and as short, separate, and independent lines of 

 classification or descent as possible. But may we look upon such groups, famiKes, or orders, 

 as separate and independent existences P or, like genera, are they merely artificial aids to memory 

 and arrangement. It appears to me that they are in many respects more real than modern genera. 



In the first place, the difierent orders (or, to prevent all dispute as to what an order really is, let 

 us say some of the different groups) are undoubtedly very homogeneous. 



" In all the instances of approach of species of one order to other orders," says Mr. Waterhouse, 

 in speaking of the classification of the different orders in one of the best of his many excellent papers, 

 " there is not a single case which would fairly bear out the notion that these orders imperceptibly 

 blend into each other. There is always a tolerably weU marked line between them. The aberrant 

 species are readily traced back as it were into their own groups, and when they evince an approach to 

 other circles it is rather to the order than to any particular species of the order."* 



In the next place, a species is something separate and distinct, which, although composed of 

 many items, is still only a unit. This, I think, is an inevitable corollary to the hypothesis that 

 new species come into existence in one body of many individuals. If we could believe the same 

 of orders we should be relieved from some of the most difficult questions attending the origin of 

 species. 



In manamals, one of these is the relation which subsists between certain Marsiipials which 

 represent in that order equivalent groups of placental Mammals, and the difficulty arises in attempt- 

 ing to ascertain the respective origin of these Dromios. The Marsupials have many claims to be 

 considered one of the oldest, if not the oldest, order of Mammals. The following table of the 

 sequence in which Mammals have appeared on the earth, shows in a general way (without 

 takmg into account disiDuted or exceptional cases) theii- relative priority, so far as geology has been 

 able to determine it. 



Geological 

 Epochs. 



Recent 



Pli( 



Miocene 



Upper Eocene 



Lower Eocene 



Secondary for- 

 mations 



Orders of Mammals. 



Man (?) 



Monkeys — Proboscoid Pachyderms (Elepliants) — Edentata, (Megatheria, &c.) 



Insectivora (Shrews, &c.) — Rodents (Rats, &c). — Ruminants — Whales 



Bats — Tapyroid Pachyderms — Carnivora 



Marsupials 



* Watekhouse, On The Classification of the Mammalia in " Annals of Nat. Hist." Vol. xii. p. 399. 1843. 



