454 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



the inertia of ordinary heredity against new modifications also weighs against the Lamarck- 

 ian hypothesis. 



The Mechanism of Correlation 



In order to account for the correlation of adaptive changes in different parts of the body 

 during the same period, e.g., correlated changes in the upper and lower jaws and teeth, in 

 the jaw muscles, supporting structures, digestive system, food preferences, etc., it would 

 therefore seem necessary to infer that some sets of physiologically connected adult charac- 

 ters are, at least to some extent, predetermined in connected parts of one genie system, and 

 that shift in the survival value in a given direction {i.e., toward shorter teeth and stronger 

 jaw muscles) would encourage the survival of individuals in which the optimum combination 

 of characters had been predetermined in one genie system. If this be so, it may be possible 

 some day to parallel the evolutionary history by artificial means, transforming the preda- 

 tory mouth of a lutianid into the crushing mouth of a sparid. 



In commenting on the foregoing section. Dr. G. K. Noble has kindly submitted the 

 following note, under the title "What Molds the Skeleton.^" 



"Twitty (1929) has shown that if the eye of A. tigrinum is transplanted into the site of 

 A. maculatum eye, the trabecula on this side would show a marked enlargement throughout 

 most of its length. Thus the eye controls the size of the trabecula. Again, the auditory 

 vesicle of Amphibia migrating from the overlying ectoderm induces the development of a 

 cartilaginous capsule about itself even when transplanted to the region of the eye (Luther, 

 1925). The cartilaginous nasal capsules of Ambystoma were shown by Burr (I9I6) to be 

 dependent upon the nasal sacs for their conformation. Hence the skull is not merely molded 

 by the paired sense organs; parts of it are unable to develop unless the sense organs are 

 present. 



"Limb rudiments of chicks have been grown in vitro, and have produced rudimentary 

 trochanters. Further, there is abundant evidence that bone architecture may be assumed 

 independent of function. Perhaps the most recent paper on this subject is Benninghoff, A., 

 1930, Morph. Jahrb., 65 ; 1 1—45. Of course function may later modify or completely reverse 

 the type of architecture, but my point that architecture can be established without function 

 seems to be definitely proved." 



Thus experimental evidence confirms the inferences derived from the present study 

 that the skeleton is molded by the soft parts, and that there are "intrinsic hereditary fac- 

 tors" in each skeletal part. 



In conclusion, my studies of the skulls of fishes suggest that the following factors or 

 conditions have been conspicuous in moulding the skull into observed types: 



I. Heredity, implying 



(a) individuality, manifested in differential growth of each skull bone; 



(b) more or less resistance to change, — general skull patterns often persisting 



through millions of years; 



(c) marked organizing ability of individual growth centers; 



(d) responsiveness, perhaps to hormones stimulating differential and successive 



growth along the three axes; regional and local responsiveness; 



