HOMOLOGY, ANALOGY AND PLASIS 



167 



the motor, the visual or the auditory 

 region respectively of the brain of man 

 and of anthropoid apes as homologous 

 each to each. They have homology of 

 position, homology of development, and, 

 as the investigations of A. W. Campbell 

 show, they have in each case essential 

 identity of intimate structure. As we 

 cannot doubt that their intrinsic cellular 

 mechanism is in each case of similar 

 character, this example also goes to show 

 that homology and Function B invariably 

 coincide. 



An additional criterion of homology. The 

 truth is, wherever we inquire, the vaguely 

 defined thing that we call homology 

 implies identity of intrinsic mechanism in 

 the homologous parts. We thus obtain 

 a new — and, be it noted, a very sharp — 

 criterion of homology, based not on posi- 

 tion nor on embryological evidence, but 

 on physiological similarity, and the test 

 of such similarity is an experimental one. 

 When Geoffroy in his notable attempt to 

 clarify the problem of morphology dis- 

 carded analogy, he was guided by a true 

 instinct. Not so, when in his ignorance 

 of the potentialities of physiology he 

 waved all physiological considerations 

 aside. His connection with the problem 

 is merely a fine example of the thesis, 

 elaborated by Sir J. G. Fraser in his 

 "Psyche," that methods and institutions 

 in themselves open to patent objection 

 may at an early phase of culture exercise 

 a very wholesome and salutary effect. 



The new criterion applied to classification. 

 It were tempting at this juncture to stop 

 and point out that in the classification 

 of animals considerations pertaining to 

 Function B can furnish significant and 

 suggestive data. Without even specify- 

 ing any of the usually cited facts of formal 

 structure one might define the group of 

 Vertebrata, for example, in terms of 

 common erythrocytic, common splenic, 



common thyroid, common pancreatic- 

 insular, common adrenal, common hypo- 

 physeal and common labyrinthine func- 

 tion; the Tetrapoda in terms of superadded 

 parathyroid and, possibly, bone marrow 

 function; the combined group of Saurop- 

 sida and Mammalia in terms of the 

 common maintenance and attachment 

 function of their stratum corneum; the 

 Sauropsida in terms of common purin 

 metabolism; the combined group of Aves 

 and Mammalia in terms of common 

 homoiothermism; the Mammalia in terms 

 of common lacteal function; the Eutheria 

 in terms of common luteal function. 

 Similarly with other animal phyla and 

 their subdivisions. The systematic text- 

 books of the future will no doubt develop 

 and extend these physiological criteria 

 of resemblance and of difference. When, 

 too, it has once become possible to inter- 

 pret the physiological significance of the 

 various combinations of structural char- 

 acters that now define the different 

 animal groups, our present systems of 

 classification will take on a new meaning. 

 For the present, however, it is more im- 

 portant to proceed a stage further in our 

 inquiry with regard to the specific problem 

 at issue. 



ONTOGENETIC AND PHYLOGENETIC FLUX 



It has been pointed out how the new 

 science of embryology introduced into the 

 problems of morphology the idea of flux. 

 No sooner had embryology become well 

 established than Darwin imported into the 

 subject still another fluxional conception. 

 The various animal species, having 

 evolved from ancestral forms dissimilar 

 to them, are not themselves fixed or 

 static entities. Rather, they represent 

 temporary phases in a continuous line of 

 progressive change. Morphology has 

 thus to take cognisance of ontogenetic 

 flux and of phylogenetic flux. 



