68 
another.—And, though these differences and resemblances 
cannot be weighed and measured, their value may be readily 
estimated ; the scale or standard of judgment, touching that 
value, being afforded and expressed by the system of classi- 
fication of animals now current among zoologists. 
A careful study of the resemblances and differences pre- 
sented by animals has, in fact, led naturalists to arrange 
them into groups, or assemblages, all the members of each 
group presenting a certain amount of definable resemblance, 
and the number of points of similarity being smaller as the 
group is larger and vicé versd. Thus, all creatures which 
agree only in presenting the few distinctive marks of ani- 
mality form the ‘Kingdom’ Antmatia. The numerous 
animals which agree only in possessing the special characters 
of Vertebrates form one ‘Sub-kingdom’ of this Kingdom. 
Then the Sub-kingdom Verresrarta is subdivided into the five 
‘Classes,’ Fishes, Amphibians, Reptiles, Birds, and Mammals, 
and these into smaller groups called ‘Orders ;’ these into 
‘Families’ and ‘ Genera;’ while the last are finally broken up 
into the smallest assemblages, which are distinguished by the 
possession of constant, not-sexual, characters. These ultimate 
groups are Species. 
Every year tends to bring about a greater uniformity of 
opinion throughout the zoological world as to the limits and 
characters of these groups, great and small. At present, for 
example, no one has the least doubt regarding the characters 
of the classes Mammalia, Aves, or Reptilia; nor does the 
question arise whether any thoroughly well-known animal 
should be placed in one class or the other. Again, there is 
avery general agreement respecting the characters and limits 
of the orders of Mammals, and as to the animals which 
are structurally necessitated to take a place in one or another 
order. 
No one doubts, for example, that the Sloth and the Ant- 
eater, the Kangaroo and the Opossum, the Tiger and the 
Badger, the Tapir and the Rhinoceros, are respectively mem- 
