LOGICAL METHOD IN BIOLOGY. 377 



of homology is the mode of innervation of an organ. Spengel * 

 reasoned that the homologies of these organs could be best estab- 

 lished by a comparative study of their modes of innervation in 

 other words, by discovering their relations to other organs known 

 to be correlated in definite ways among themselves. In this way 

 he succeeded in proving their morphological identity, although 

 the belief that they are olfactory organs is based simply on the 

 morphological fact that they invariably occupy a certain position 

 in relation to the respiratory organs, and not on any physiologi- 

 cal data. 



He demonstrated the general occurrence of this particular 

 kind of organ in the prosobranch gastropods, inferred that it 

 ought to occur among the opisthobranchs, and succeeded in dem- 

 onstrating its presence in the division of tectibranchs. He had 

 already in his possession the hypothesis that the organ is one be- 

 longing to the mollusca as a whole and drew from it the deduc- 

 tion that it ought to be present in the lamellibranchs, among 

 which it had not been hitherto known. He said : " The position in 

 which such a one would have to be sought was clearly enough 

 indicated to me by rny observations on the gastropods. It would 

 have to be in the neighborhood of one of the ganglia of the vis- 

 ceral commissure." Trusting this definite anticipation, he looked 

 for the olfactory organ and found it in Area Now, the first mus- 

 sel he opened for the purpose. In this species the organ is char- 

 acterized by pigment, which made its recognition easy. In other 

 species that he examined the pigment is absent, and had he first 

 opened one of these, he might have had a long and possibly fruit- 

 less hunt for the organ. This well illustrates how important a 

 part chance frequently plays even in deductive investigation. It 

 is interesting to note how the deduction might have remained un- 

 verified and possibly have been adandoned and yet have been a 

 true one. 



The organ typically consists of thickened epithelium innervated 

 from a ganglion underlying it. Theory required the presence 

 of a ganglion under the olfactory organ of lamellibranchs, but 

 there was apparently only a strong nerve, which had hitherto 

 been universally interpreted as the "gill nerve." Histological 

 examination proved it to be an elongated ganglion inserted on 

 the nerve between its origin and its ending in the gill. Here 

 again, a deduction led to a discovery and the correction of what 

 had seemed for years to be a settled fact. 



Spengel had shown, in his study of other groups, that the 

 nerve on which the olfactory ganglion lies arises from the vis- 



* Die Geruchsorgane und das Nervensystem der Mollusken. Zeitschrift fur wissen- 

 schaftliche Zoologie (April 22, 1881), vol. xxxv, pp. 333-383. 



