444 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



which tell us whether the fish was typically either predaceous or a plankton feeder, a 

 "nibbler," a "pincers" fish, a crusher of bivalves and crustaceans, a mud-grubbing, sucking 

 type, or an animated fish-trap. Checking of these inferences by examination of stomach 

 contents in members of the same species would of course be desirable. Then we look at the 

 general body-form for the marks of swift pelagic swimming, of the sudden rushes of a lurking 

 robber, of the quick dodging by a deep-bodied fish of the coral reefs, etc. 



As to how far one can read the ancient lineage of a fish from study of skeletal characters, 

 it is well known that if the skeleton is well preserved there is no difficulty in detecting diag- 

 nostic characters, including proportional lengths of various parts by which to determine 

 the systematic position of the specimen. This in turn would enable us to look up whatever 

 general palseontological or taxonomic evidence there might be as to the steps by which the 

 group under consideration acquired its peculiar characters. Unfortunately, perhaps 

 ninety per cent or more of the energies of ichthyologists have been expended in determining 

 the marks of new species rather than in endeavoring to discover the stages by which a given 

 species has come by its present characteristics. 



In connection with inquiries into the evolutionary history of any given type of fish 

 skull, it is important to realize that the habitus (or totality of hereditary adaptations to a 

 given way of life) in the ancestor becomes the phylogenetic "heritage" of its descendant 

 (Gregory, 1913). For example, the predaceous habitus of the ancestral percoid becomes 

 the phylogenetic heritage of such specialized forms as the beaked parrot wrasses, the 

 nibbling balistids, the trap-mouthed anglers and many others. In other words, the earlier 

 functions and structures of the predaceous habitus had to be modified progressively away 

 from this relatively primitive condition; but traces of these earlier habitus characters may 

 still be seen in many basic features of the branchiocranium and neurocranium of even the 

 most specialized teleosts. In the light of these facts and principles we have already gained 

 a probably fairly accurate historical concept as to the main steps (see pp. 85, 416) by 

 which some remote prechordates that fed by ciliary ingestion were transformed into the 

 central predaceous type of teleost fish, and as to the subsequent steps by which this fish in 

 turn was changed into such diverse types as "nibblers," "pincers," "crushers," "tube- 

 mouths," "scoop-nets" and so forth. 



From all this emerges the generalization that even on the unlikely hypothesis that 

 structural changes preceded new feeding habits and merely made possible the more thor- 

 ough exploitation of possible food sources, there were in each phylogenetic series a large 

 group of changes, afi"ecting many parts of the skull and body that were correlated with 

 each other, and that every evolutionary change in structure implied corresponding changes 

 in food preferences, involving shifts in subtle and intricate correlations of sensory stimuli 

 and motor response. 



The Mechanism of Regulation 



Accordingly we are led to inquire as to the physiological mechanism for regulating the 

 size-relations of various parts of the body. The measurements made by systematists on 

 thousands of species establish the fact that the ratios of head-length to body-length, of 

 preorbital length to head-length, and many others, vary only within relatively narrow limits 

 in adults of a given species. So that the species owes its characteristic contours and all its 

 specific adult patterns to a specific mechanism for the regulation of differential growth rates. 



From the fact that in certain cases, such as the hook-jawed salmon and the eel-pout, 



