102 BIOLOGICAL LECTURES. 



With the development of the " biogenetic law" or recapitu- 

 lation theory as a corollary to the general theory of evolution 

 the matter appeared in a different light. For if ontogeny be 

 in truth a repetition or record of phylogeny, then the embryo- 

 logical development must furnish the highest criterion of 

 homology, since community of phyletic origin (homology) 

 becomes synonymous with community of ontogenetic origin. 

 Now, although no one believes that ontogeny is actually a 

 true and complete record of phylogeny, it is nevertheless sur- 

 prising to observe to what an extent the embryological criterion 

 has superseded the anatomical, and how deeply rooted the 

 ontogenetic conception of homology has become, notwithstand- 

 ing all that has been written about " falsification of the record," 

 cenogenesis, secondary and adaptive modification, and the like. 

 Even the greatest of naturalists has tacitly allowed it to pass ; 

 for in the glossary of the Origin of Species (drawn up by Mr. 

 Dallas but approved by Darwin) homology is defined as : " That 

 relation between parts which results from their development 

 from corresponding embryonic parts, either in different ani- 

 mals, as in the case of the arm of man, the fore-leg of a quad- 

 ruped, and the wing of a bird, or in the same individual, as in 

 the case of the fore and hind limbs in quadrupeds, and the seg- 

 ments or rings and their appendages of which the body of a 

 worm, a centipede, etc., is composed." It appears in the 

 writings of many eminent contemporary morphologists and 

 has crept into some of our best dictionaries. I cite a single 

 example of each : " On the other hand, the point of funda- 

 mental importance in the comparison of two or several organs 

 is their origin from equivalent parts of the embryo." "Hence 

 in the comparison of two or more embryonic organs we must 

 rely not so much upon their later development as on their first 

 origin." 1 The Century Dictionary has it thus: Homology is 

 " that relation of parts which results from their development 

 from corresponding embryonic parts. Homology in this sense 

 implies genetic relationship, and consequently morphological 

 likeness or affinity " z>., the morphological likeness is an 

 inference from the mode of ontogeny ! 



1 Rabl: Theorie des Mesoderms, I. Morph. Jahrb., XV, 1889, p. 229. 



