HOMOLOGY, ANALOGY AND PLASIS 



157 



law," according to which the ontogenetic 

 development exhibits an abbreviated repe- 

 tition of phylogenetic history, in other 

 words that individual embryonic stages 

 represent reanimated but schematised 

 ancestors of the fully developed animal. 

 If now two apparently diverse organisms 

 are found to be structurally comparable, 

 part for part, in the adult stage, will not 

 their individual homologies be rendered 

 more evident by examination of their 

 development (which, by hypothesis, 

 mirrors their phylogeny) before speciali- 

 sation or extreme differentiation has 

 occurred? Particularly apposite evidence 

 in support of this notion was already to 

 hand from the science of embryology. 

 Consequently we find Haeckel announcing 

 that "true homology can only exist 

 between two parts which have arisen 

 from the same primitive 'Anlage' 

 (embryonic representative) and have de- 

 viated from one another by differentiation 

 only after the lapse of time." 



For the moment the old positional or 

 geometrical conception of homology had 

 faded into the background. Gegenbaur's 

 definition of special homology, viz., 

 "the relationship between two organs 

 which have had a common origin, and 

 which, as a corollary, have arisen from the 

 same Anlage," incorporates to the full 

 the biogenetic law of Haeckel. The 

 new interest attaching to the establish- 

 ment of ' 'homologies ' ' was thus expressed 

 by Gegenbaur: "Homology .... cor- 

 responds to the hypothetical genetic re- 

 lationship. In the more or less clear 

 homology, we have the expression of 

 the more or less intimate degree of rela- 

 tionship. Blood-relationship becomes 

 dubious exactly in proportion as the proof 

 of homologies is uncertain." 



How now does the amended criterion 

 of homology work out in practice? For 

 one thing the old bugbear of analogical 



resemblance or similarity by adaptation 

 was not yet lightly to be exorcised, and 

 we find Haeckel remarking: "In evalua- 

 tion of anatomical resemblances .... 

 everything in the last issue invariably 

 depends on the decision whether the 

 ultimate correspondences in structure are 

 to be looked upon as homologies (main- 

 tained by common descent) or as analogies 

 (acquired by similar adaptation). Im- 

 portant as it is, this very decision is often 

 extremely difficult." Another perplexity 

 inherent in the new definition was that 

 which occurs to so many young students 

 of morphology, namely, of fixing a limit 

 of commencement to the "Anlage" of an 

 adult organ; for a group of such "Anlagen" 

 may have themselves a common "Anlage" 

 in an earlier period of development; in 

 this way one arrives at the blastomeres 

 and finally at the ovum itself. Partly as 

 a correction against this perfectly per- 

 tinent but perfectly useless extension of 

 the embryological criterion — which, inter- 

 preted phylogenetically, would make 

 almost all organs of any one phylum 

 homologous one with another — and partly 

 because of greater convenience in practice, 

 the evolutionary morphologists, while 

 introducing the embryological conception 

 into their definition of homology, con- 

 tinued in the habit of establishing homolo- 

 gies by comparison of adults, just as the 

 older anatomists did. It has ever proved 

 more easy to alter a creed than to uproot 

 long-established usages associated with an 

 older faith. While the morphologists 

 professed embryological homology, their 

 conduct alternated between the homology 

 that depends on simple position and that 

 which is defined in terms of similarity of 

 development. 



If we inquire as to the place allotted to 

 function in the new order of things, we 

 find that it was consistently discussed in 

 the sense of use or application (Function 



