In a perfect taxonomy, or natural system of classification, animals would not be divided into groups nor ranged in linear series. We should imagine a series variously and divergently branched, with each group at its earlier or lower end passing insensibly into the main or primitive stock. A very little alteration now and then in some structure is epoch-making and paves the way through specialization to a new class or order. But each class or order through its lowest types is intertangled with some earlier and otherwise diverging group. A sound system of taxonomy of fishes should be an exact reflex of the history of their evolution. But in the limitations of book making, this transcript must be made on a flat page, in linear series, while for centuries and perhaps forever whole chapters must be left vacant and others dotted everywhere with marks of doubt. For science demands that positive assertion should not go where certainty can not follow.
A perfect taxonomy of fishes would be only possible through the study, by some Artedi, Müller, Cuvier, Agassiz, Gill or Traquair, of all the structures of all the fishes which have ever lived. There are many fishes now living in the sea which are not yet known to any naturalist. Many others are known to one or two, but not yet accessible to those in other continents. Many are known externally from specimens in bottles, or drawings in books, but have not been studied thoroughly by any one, and the vast multitude even of the species have perished in Paleozoic, Mesozoic and Tertiary seas, without leaving a tooth or bone or fin behind them. With all this goes human fallibility, the marring of our records, such as they are, by carelessness, prejudice, dependence and error. Chief among these are the constant mistakes of analogy for homology, and the inability of men to trust their own eyes as against the opinion of the greater men who have had to form their opinions before all the evidence was in.
The result is, again to quote from Dr. Coues:
It is obvious that fishes, like other animals, may be classified in numberless ways, and, as a matter of fact, by many different men they have been classified in all sorts of fashions.
