126 ON THE HOMOLOGIES OF CERTAIN MUSCLES 



muscle of the same origin as the smaller levator of the fowl ; and 

 we have thus an intermediate stage between the trifid insertion of 

 the levator-apparatus of the fowl and the single tendon of the hawk. 

 It will be obvious that the 'deltoides externus' of the common 

 fowl corresponds, both as to origin and insertion, with the anterior 

 or praeglenoid head of the ' epicoraco-humeral ' of the crocodile, and 

 that its smaller levator humeri corresponds as closely, as to its inser- 

 tion, with the postscapular head of that reptilian muscle ; and it 

 might seem, therefore, that the larger levator humeri muscle of the 

 fowl remains without a separate homologue in the reptile, were not 

 this muscle innervated by the same subclavius nerve as the reptilian 

 epicoraco-humeral, whilst the smaller levator humeri is, together 

 with the deltoides externus, innervated from the circumflex. To an 

 objector who denies the validity of this argument, and lays stress 

 on the fact that the reptilian muscle has not the sternal factor of 

 the avian pectoralis secundus, I reply, first, in the emu a muscle 

 very unmistakeably homologous with the praeeoraco-scapularis of 

 the crocodile possesses but a very small sternal factor; and, 

 secondly, in the ostrich (Struthio camelus), which also retains a 

 pectoralis secundus, this sternal factor is aborted and lost. Stages 

 of successive degradation from a highly specialised structural 

 arrangement may well serve to guide our judgments in problems 

 such as these, if it be true that the essential elements of a structure 

 are the last which it loses so long as it retains any actual func- 

 tional power at all. But, thirdly, lower types than even the scaly or 

 loricate reptile may have an epicoraco-humeral with a sternal factor. 

 This is the case in the frog, where the epicoraco-humeral (the ' pars 

 clavicularis deltoidei ' of Ecker, ' Die Anatomie des Frosches,' p. 97) 

 arises from what Mr. Parker (' Shoulder-Girdle,' pp. 79, 80, and 

 pi. v.) has shown to be the praecoracoid and the omosternum. 



With this I close my case for proving that the epicoraco-humeral 

 of the reptile is the homologue of the pectoralis s. levator humeri 

 of the bird. It is not necessary for the acceptance of this conclusion 

 that a person should accept also the Theory of Progressive Evolution. 

 By speaking of organs as being ' homologically identical/ a person 

 who is either averse to pronounce himself, or anxious to secure 

 a dispassionate consideration from all parties, or both, may avoid 

 pledging himself either to the theory which teaches that they are 

 so by virtue of the 'secret bond' of their ' conformity to type,' or to 



