character or group of characters which may be relied on for systematic purposes in all groups of animals. Certain characters of remarkable constancy or obvious significance in some groups are quite unreliable in others. Color is a conspicuous illustration of this important fact, for in certain groups it is very constant, or has some very evident significance, while in others it is so inconstant and diversified that it is of no use to the systematist. And this remarkable difference in the value of color may occur within the limits of a single genus. Only by extended knowledge of a group can it be determined what characters are of value for either the differentiation or the grouping of species. It is perfectly proper to speak of questioning the validity of a species and its validity must ultimately be determined by the constancy of the character or group of characters supposed to distinguish it. But it is not correct to speak of questioning the "validity" of a genus. Its desirability and the accuracy of its definition may either or both be questioned. One genus is as "valid" as another but the desirability of naming any group is a matter of opinion. And this brings us to the last of the principles I wish to formulate.
In all systematic work, the line between facts of nature and opinions of the worker should be sharply drawn; the value of the work often depends on the clearness of this line.
One of America's greatest zoologists was wont to repeat over and over again to his students these words: "The assertion that outstrips the evidence is a crime." Like most aphorisms of the kind this sentence needs some qualifications, but zoologists will hardly question the modified form that "the assertion as a fact, of what is really only an opinion, is a crime." The more unqualified the assertion, or the hazier the opinion, the greater is the crime. The opinions of a writer, particularly if based on careful observations and long experience, may be as valuable as his facts, but it ought never to be possible to confuse the two. It should be the aim of every systematic zoologist to set forth his facts so distinctly and so unmixed with opinions that any qualified worker may form accurate opinions for himself, and to so express his opinions that the justification for them in the facts of nature may be clear to all.
Having thus set forth what seem to me five of the most important principles of systematic zoology and realizing the possibility of varied shortcomings of my own with reference to them, I can only add in conclusion, as indicative of my repentance, Peccavi.