HISTORICAL INTERPRETATION OF FORM 309 



could no longer be explained idealistically as the manifesta- 

 tion of Divine archetypal ideas ; it had a real historical 

 basis, and was due to inheritance from a common ancestor. 

 The evolution-theory gave meaning and intelligibility to the 

 transcendental conception of the unity of plan ; in particular 

 it supplied a simple and satisfying explanation of those 

 puzzling vestigial organs, whose existence was such a 

 stumbling-block to the teleologists. It enabled the biogenetic 

 law to be substituted for the laws of Meckel-Serres and von 

 Baer, as being in some measure a combination and interpre- 

 tation of both. 



Where the concept of evolution proved itself particularly 

 useful was in the interpretation of structures which were not 

 immediately conditioned by adaptation to present require- 

 ments, such as, for instance, the arrangement of gill-slits and 

 aortic arches in the foetus of land Vertebrates. Such " heritage 

 characters " could only be explained on the hypothesis that 

 they had once had functional or adaptational meaning. 

 Why, for instance, should the blastopore so often appear as 

 a long slit, closing by concrescence, unless this had been the 

 original method of its formation in remote Coelenterate 

 ancestors ? 



The point hardly requires elaboration, since it has become 

 an integral part of all our thinking on biological problems. 

 It may be as well, however, for the sake of continuity, to 

 give one or two examples of the historical interpretation of 

 animal structures. The first may conveniently be the phylo- 

 genetic interpretation of the contrast between " membrane " 

 and "cartilage" bones. 



In his Grundzuge of 1870, Gegenbaur made the suggestion 

 that the investing or membrane bones were derived phylo- 

 genetically from integumentary ossifications, and this was 

 worked out in detail a few years later by O. Hertwig. 1 



Many years before, several observers J. Miiller, William- 

 son, and Steenstrup had been struck with the resemblance 

 existing between the placoid scales and the teeth of Elasmo- 

 branch fishes. Hertwig followed up this clue, and came to 

 the conclusion not only that placoid scales and teeth were 



1 Arch. f. mikr. Anat., xi. (suppl.), 1874; Morph. Jahrb., ii., 1876, 

 v. 1879, an< 3 vii., 1882. 



