ceral ganglia inserted in the visceral commissure, and not from the pleural ganglia. With one stroke of deduction he swept away a whole brood of old views. He reasoned that the parietosplanchnic ganglia of lamellibranchs, from which this nerve arises, must on account of this very fact be the visceral ganglia, and not what they had been universally assumed to be—the homologues of the pleural ganglia of gastropods. The old view necessitated the belief that the renal, reproductive organs, etc., of lamellibranchs are innervated from the pleural ganglia, and that the foot with all its accessories is included within the œsophageal ring of ganglia; whereas in other mollusks the renal, reproductive, and associated organs are innervated from the visceral ganglia and the foot lies outside of the œsophageal ring. If the parieto-splanchnic ganglia of lamellibranchs are homologous with the visceral ganglia of other mollusca all the above-mentioned organs hold the same relations in lamellibranchs as in the other groups. This reinterpretation of so many known facts harmonizes the lamellibranch type completely with that of the general molluscan type and marks a distinct step in the progress of molluscan morphology. He pursued a similar though less complete course with the cephalopods.
By this method of morphological reasoning, accompanied and corroborated or corrected at every step by morphological investigation, a heterogeneous mass of facts was bound together under the principle of homology, and many new ones were discovered that would not have been brought into notice in any other way. Indeed, the principle of homology, together with the principle on which it depends, the correlation of organs, furnishes a basis without which it would be nearly impossible to make intelligent search for new facts. Incessant use is made of the general logical principle that things that are similar in some respects, are likely to prove similar in other and unknown respects, and that things similar in many respects are likely to prove similar in most or all respects, in anticipating biological facts. It is well known that many of the facts of greatest theoretical importance in biology have been overlooked until hypothesis pointed them out. Yet this power of prevision is one of the most dangerous of pitfalls. No rule can be laid down for the use of the principle, because there is none. There is a general precaution to be observed: similarity in a few respects is no warrant for inferring similarity in many respects, much less all respects. Too many biologists, among them some of the most eminent, seem to have a wrong conception of the function of this logical principle. Scholastic methods are the favorite butt of scientific wit, but that notorious old tendency to speculate without due regard to facts is not dead but only facing in another direction. The stupid blunders and