It is said, in short, that a natural-history class is not capable of being defined--that the class Rosaceae, for instance, or the class of Fishes, is not accurately and absolutely definable, inasmuch as its members will present exceptions to every possible definition; and that the members of the class are united together only by the circumstance that they are all more like some imaginary average rose or average fish, than they resemble anything else. But here, as before, I think the distinction has arisen entirely from confusing a transitory imperfection with an essential character. So long as our information concerning them is imperfect, we class all objects together according to resemblances which we _feel_, but cannot _define_: we group them round _types_, in short. Thus, if you ask an ordinary person what kinds of animals there are, he will probably say, beasts, birds, reptiles, fishes, insects, &c. Ask him to define a beast from a reptile, and he cannot do it; but he says, things like a cow or a horse are beasts, and things like a frog or a lizard are reptiles. You see _he does_ class by type, and not by definition. But how does this classification differ from that of the scientific Zoologist? How does the meaning of the scientific class-name of "Mammalia" differ from the unscientific of "Beasts"? Why, exactly because the former depends on a definition, the latter on a type. The class Mammalia is scientifically defined as "all animals which have a vertebrated skeleton and suckle their young." Here is no reference to type, but a definition rigorous enough for a geometrician. And such is the character which every scientific naturalist recognises as that to which his classes must aspire--knowing, as he does, that classification by type is simply an acknowledgment of ignorance and a temporary device. So much in the way of negative argument as against the reputed differences, between Biological and other methods. No such differences, I believe, really exist. The subject-matter of Biological science is different from that of other sciences, but the methods of all are identical; and these methods are-- 1. _Observation_ of facts--including under this head that _artificial observation_ which is called _experiment_. 2. That process of tying up similar facts into bundles, ticketed and ready for use, which is called _Comparison_ and _Classification_,--the results of the process, the ticketed bundles, being named _General propositions_. 3. _Deduction_, which takes us from the general proposition to facts again--teaches us, if I may so say, to anticipate from the ticket what is inside the bundle. And finally-- 4. _Verification_, which is the process of ascertaining whether, in point of fact, our anticipation is a correct one.