378 THE POPULAR SCIENCE MONTHLY. 



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 parieto- 

 splanchnic 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 

 oesophageal ring of ganglia ; whereas in other mollusks the renal, 

 reproductive, and associated organs are innervated from the vis- 

 ceral ganglia and the foot lies outside of the oesophageal ring. 

 If the parieto-splanchnic ganglia of lamellibranchs are homolo- 

 gous 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 investi- 

 gation, 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 logi- 

 cal 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 biol- 

 ogy 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 con- 

 ception 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 



