36 The Science of Life. 



be thought of for a moment, since many of the bones 

 (e.g. along" the top of the skull) arise in the skin. 



Gegenbaur has been a powerful exponent of the idea 

 that new structures do not arise de novo, but from 

 alterations in pre-existing structures. Thus he has 

 been a supporter of the theory that the limbs of verte- 

 brates have arisen from an alteration in the position 

 and function of some of the branchial or visceral 

 arches whose original use was to support gills. 



As another instance we may refer to the musculature 

 of the tongue. This does not occur in fishes, whose 

 tongues are all non-muscular. The mobility begins in 

 amphibians, and Gegenbaur has shown that the muscles 

 are at first too small and weak to be used to move the 

 member. They serve in the young tadpole merely to 

 compress the glands of the tongue, but they grow in 

 strength and take on a new function which has been of 

 great importance to amphibians and higher animals. 



The basis of a natural classification, and what comes 

 to the same thing, a probable pedigree, has been found 

 in the recognition of homologous structures in different 



Criteria of organisms. It is therefore of great import- 



Homoiogy. ance that the homologies be secure, and it is 

 distinctive of modern morphology that the question of 

 the criteria of homology is not treated in the easy-going 

 fashion that was for a time prevalent. 



Historically, the case stands thus. To Owen, hom- 

 ology meant anatomical correspondence in the relative 

 position and connections of parts. Gradually the 

 anatomical correspondence found embryological cor- 

 roboration, and this was most welcome. But the 

 modern enthusiasm for embryology and the influence of 

 the Recapitulation Doctrine have led to a predominance 

 of the embryological, and a partial superseding of the 

 anatomical criteria. This has often given rise to a 

 wildness of speculation as to pedigrees (phylogeny) 

 which leaves the anatomist bewildered. 



From this exaggerated confidence in the embryo- 

 logical revelation of relationships, the inevitable re- 

 action has ensued. Thus Prof. E. B. Wilson gives 

 many examples which show that "embryological de- 



