GREGORY: FISH SKULLS 449 



run to variable chemical and physical influences, especially during the sensitive periods of 

 the division of the egg and the formation of the Anlagen of the adult skull. 



The objection that Natural Selection does not originate individual changes and there- 

 fore does not account for phylogenetic differences between descendants and ancestors could 

 be reconciled with available evidence of the evolution of the fish skull only if we quite 

 arbitrarily limit Natural Selection to mean solely the selective results of competition of one 

 fish with another. But if we use Natural Selection, as Darwin did, as a sort of personifica- 

 tion of the vast complex of active forces and passive conditions which cumulatively result 

 in hereditary differences between descendants and ancestors, then we can recognize that 

 such cumulative actions, reactions and interactions have manifestly produced a wide 

 diversity of efi"ects upon the now scattered descendants of the primitive percomorph skull 

 type. In some lines Selection has evidently penalized departure from the primitive skull 

 patterns of various grades and has conserved for scores of millions of years the various 

 skull types now exhibited by Amia, Lepidosteus, Erythrinus, Megalops, Gonorhynchus, 

 Alepisaurus, Percopsis, Beryx, Centrarchus, Serranus, etc., each of which is a primitive or 

 central type in its own ordinal group. In other cases "Natural Selection" has encouraged 

 a high tendency to hereditary variability so that the skull patterns have lost the primitive 

 percomorph type and assumed many new disguises to such a degree as to bafiie the best 

 efforts of several generations of investigators, who have sought in vain to determine the 

 precise relationships of gobies, gobiesocids, echeneiids, symbranchoids, mastacembelids 

 and others. In short the present study, which combines field observations on the varied 

 functions of the fish skull, museum studies on the structure of recent and fossil fish skulls 

 and the chief results of the literature of the subject, is submitted in evidence of the power 

 of Natural Selection to produce wide secular differentiation among the descendants of a 

 never entirely stable ancestral germ plasm. 



