Startling as the last assertion may appear to be, it is demonstrably true, and it alone appears to me sufficient to place beyond all doubt the structural unity of man with the rest of the animal world, and more particularly and closely with the apes. * * * * * Thus, identical in the physical processes by which he originates--identical in the early stages of his formation--identical in the mode of his nutrition before and after birth, with the animals which lie immediately below him in the scale--Man, if his adult and perfect structure be compared with theirs, exhibits, as might be expected, a marvellous likeness of organization. He resembles them as they resemble one another--he differs from them as they differ from one 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 classification of animals now current among zoologists. A careful study of the resemblances and differences presented 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 _vice versâ_. Thus, all creatures which agree only in presenting the few distinctive marks of animality form the "Kingdom" ANIMALIA. The numerous animals which agree only in possessing the special characters of Vertebrates form one "Sub-kingdom" of this Kingdom. Then the Sub-kingdom VERTEBRATA 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 a very 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.