HOMOLOGY, ANALOGY AND PLASIS 



169 



kind that is entirely new, and this they 

 have done over and over again. 



While no one would dispute that the 

 science of morphology has been erected on 

 the conception of homology, it is also 

 clear that the help obtainable from 

 homology in any study of animal structure 

 is of a restricted kind. Homology is at 

 best an inference originally forced upon 

 comparative anatomy by its material. 

 It is an inference that arises mainly from 

 similarities; from dissimilarities only in 

 so far as these serve as a background or 

 contrast to the similarities. Practically 

 speaking, it is an effective method of 

 classification. It serves to disentangle 

 any large assemblage of forms and to 

 group structurally similar forms under 

 common categories. Rationally speak- 

 ing, it does not in any way explain the 

 similarities which gave it birth. Even 

 for purposes of classification it has been of 

 assistance only in laying down the broad 

 features of similarity of the larger groups. 

 It has been of little real service in assessing 

 the nature of the gaps between widely 

 divergent types . For finer familial, generic 

 and specific distinctions it is scarcely ever 

 used, because it does not profess to deal 

 with dissimilarities. At the same time 

 our whole knowledge of evolution, which 

 is fundamental for any science of animal 

 structure, teaches us that a critical phase 

 of the evolutionary process lies in the 

 origin of varieties and of new species. 



Because homology, and for that part 

 analogy, deal only with similarities, they 

 are both defective principles in the face of 

 developing diversity. As divergences 

 arise not discontinuously but by slow 

 gradation (we are now speaking in 

 phylogenetic terms and are not raising 

 the issue of mutations, which from our 

 present point of view is a secondary 

 matter), it is clear that simple static 

 homology and analogy do not provide any 



infinitesimal calculus that can be applied 

 to the question of flux, i.e. to the birth 

 and continuous development of dissimi- 

 larity. If the systematic study of structure 

 is to go on advancing, homology must be 

 reinforced, analogy, in its limited field of 

 operation but from the plastic point 

 of view, must receive its full meed of 

 consideration, while attention must be 

 directed to differences as much as to 

 similarities. 



Experimental embryology has already 

 applied itself to the problem of onto- 

 genetic flux. Except by implication it 

 has not touched upon phylogenetic flux. 

 We are aware that phylogenetic flux in 

 some way dominates and sets the character 

 of ontogenetic flux. So far as one can 

 say, it is primary, ontogenetic flux being 

 secondary. For the intimate study of 

 phylogenetic flux the one ready instru- 

 ment to our hand is physiology. An 

 example or two may suffice to illustrate 

 the matter. 



The sulcus lunatus of the Chinese brain. 

 By gross examination of the sulci and of 

 the sectioned cortex, Shellshear has 

 recently shown that the occipital region 

 of the Chinese differs as a rule from that 

 of the European brain. In the Chinese 

 brain a clearly marked sulcus lunatus or 

 "Affenspalte" extends well out on to the 

 lateral surface of the occipital lobe. In 

 most cases its posterior lip forms an 

 operculum, and the stria of Gennari, 

 which characterises the "visuo-sensory" 

 region of Bolton, extends to the posterior 

 lip of the sulcus. Thus in the Chinese 

 brain the stria of Gennari, instead of 

 being limited to the mesial aspect of the 

 occipital lobe as in Europeans, spreads on 

 to the lateral aspect. It is a common 

 saying among the Chinese themselves that 

 their vision differs from that of Europeans. 

 In looking directly forward they claim to 

 have a clearer perception of what is 



