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THE QUARTERLY REVIEW OF BIOLOGY 



The homology conception arose when 

 animal forms were believed to be fixed 

 or static. The introduction of embry- 

 ology failed in any way to modify the 

 conception of homology. The embry- 

 ological criterion of homology was not 

 due to embryology as such. 



With the advent of Darwinism a for- 

 ward step was made. In its implications 

 for morphology ontogenetic flux was now 

 for the first time taken into practical 

 account. An attempt, confused but still 

 definite, was made to incorporate "embry- 

 ological homology" as a method into the 

 working equipment of the subject. The 

 recognition of phylogenetic flux led at 

 first to no new practical method of its 

 own, just as the introduction of embry- 

 ology on a previous occasion led to no new 

 morphological method. 



Lankester's effort was both critical and 

 constructive. His homogeny is static, 

 much as the primitive positional ho- 

 mology was static. The recognition of 

 plasis, limited as was the use he made of 

 the idea, is his important contribution to 

 the conception of organic flux, and in the 

 very general terms of its formulation his 

 homoplasy is applicable both to onto- 

 genetic and to phylogenetic flux. Later 

 came Entwicklungsmechanik with its strictly 

 ontological experiments, which on onto- 

 logical territory have confirmed and 

 rendered much more definite Lankester's 

 hypothesis. 



With it all, we still lack a means of 

 treating phylogenetic flux, which, be it 

 noted, is the great outstanding problem 

 of formal morphology. In this type 

 of flux we recognize the occurrence of 

 two quite different things. One is modi- 

 fication of existing structures by way of 

 adaptation to new uses; throughout these 

 modifications the intrinsic mechanism 

 remains essentially the same and the 

 parts remain homologous. The other is 



the invention or evolution of structures of 

 a quite new type, whose intrinsic mech- 

 anism is also new. It is true, the new 

 structures, which show novel biophysical 

 or biochemical behavior, themselves grow 

 out of the old. While the process by 

 which they are produced is entirely 

 obscure, their apparition denotes a 

 different kind of event from any simple 

 successful application of an existing struc- 

 ture or system of existing structures to a 

 new purpose. 



Distinct as the foregoing two problems 

 are — that, namely, of modification by 

 simple adaptation and that of brand new 

 invention — both are problems of plasis 

 in a wide sense. It should always be kept 

 in mind that homoplasis or analogy 

 is only one limited aspect of a more 

 comprehensive plasis. Analogy, not- 

 withstanding its great and unexploited 

 potentialities for the study of plasis, is, 

 so to speak, an accidental affair. Only 

 in certain cases will the modifications 

 achieved in two different forms closely 

 resemble each other. 



Exclusive preoccupation with the 

 homology method has been responsible 

 for a certain narrowness of vision among 

 those who handle animal structure. 

 There has been a tendency to consider 

 animals merely as ringing the changes on 

 a fixed or preordained equipment of 

 structure. Bateson was the first to push 

 this philosophy to its logical conclusion 

 and to assert, with what degree of con- 

 viction no one can quite make out, that 

 evolution proceeds by dropping of char- 

 acters rather than by acquisition of new 

 characters. Of course, this whole phil- 

 osophy is wrong. The inventiveness of 

 animals is not exhausted in the extra- 

 ordinary way in which they can bend or 

 mould existing equipment to new uses. 

 They can equally well, departing from 

 precedent, hammer out inventions of a 



