HOMOLOGY, ANALOGY AND PLASIS 



163 



the following sentence shows: "Further, 

 if, instead of similar parts in the same 

 organism, we suppose the same forces to 

 act on parts in two organisms, which parts 

 are exactly or nearly alike and sometimes 

 homogenetic, the resulting correspond- 

 ences called forth in the several parts in 

 the two organisms will be nearly or 

 exactly alike." Not the least striking 

 feature of Lankester's homoplasy is that, 

 enunciated in 1870, it should now have 

 been so aptly confirmed by experimental 

 work. 



Going farther than Haeckel, who gave 

 expression to a similar difficulty, Spemann 

 says, "In point of fact, when one probes 

 more closely, it is perhaps in the rarest 

 cases possible to decide with certainty 

 whether homology or analogy is in- 

 volved." Here of course he refers, not 

 to the structural similarities between 

 functionally equivalent organs of widely 

 different forms (the classical cases of 

 analogy), but to the multitude of finer 

 structural correspondences between the 

 parts of more closely related forms. 

 While his statement is a qualified one, it 

 is of interest as representing the new 

 outlook that comes of experimental hand- 

 ling of the problems of bodily structure. 

 A mere assertion of homogeny no longer 

 suffices to account for a structural corre- 

 spondence. Homoplasy, all unsuspected, 

 may have been simultaneously at work. 



When Spemann finally concludes that 

 "the homology conception as understood 

 during the historic period goes to pieces 

 in our hands when we try to work with it 

 in the causal sphere," he means simply* to 

 say that, notwithstanding all its past 

 achievements, as an engine of further 

 investigation it requires now to be supple- 

 mented by something more definite and 

 •determinate, something we can grip with- 

 out the feeling that the instrument we 

 hold is an insecure or wobbly one. No 



one will doubt that for preliminary, pro- 

 visional determination of the major rela- 

 tionships of animals one to another, the 

 conception of homology has been of 

 incalculable service. It is when we come 

 to examine into the mechanism under- 

 lying similarity or divergence, as the 

 case may be, that the need of some further 

 means of assistance becomes felt. 



HOMOLOGY AND FUNCTION B 



As we began the subdivision of this 

 article by discussing physiological con- 

 siderations, so it is proper that we should 

 return to physiology before its conclusion. 

 It will have become apparent that hom- 

 ology and similarity as regards Function 

 B as a rule coincide. The question now 

 arises: Do they always coincide? As test 

 cases let us consider some examples. 



The thyroid. Throughout the Vertebrata 

 proper (Pisces, Amphibia, Reptilia, Aves, 

 Mammalia) this ductless gland has the 

 same essential structure, consisting of 

 closed vesicles, in which a single layer of 

 epithelium as a rule surrounds colloid 

 material. Throughout the group it pre- 

 serves the same topographical position 

 and has the same embryological history, 

 viz. it arises as a median ventral down- 

 growth from the pharyngeal entoderm 

 between the first and second branchial 

 pouches, i.e., about the level of the first 

 aortic arch. Its functional attributes are 

 equally uniform and distinctive. It shows 

 a great avidity for iodine introduced into 

 the circulation; it elaborates a highly 

 specific substance, thyroxin, which con- 

 tains iodine in organic combination and 

 whose chemical constitution has not been 

 completely elucidated. In its peculiar 

 avidity for iodine and in the manufacture 

 of thyroxin, its Function B may (in a 

 measure) be said to be determined, for, 

 apart from scattered portions of similar 

 and similarly derived tissue in the region 



