DISCUSSION: THE FISH SKULL AS A DOCUMENT OF EVOLUTION 



Development and Evolution 



From all that precedes it is evident that a fish is a natural machine which inherits 

 sufficient latent energy from its parents to begin its own career of capturing, storing and 

 spending energy and of preparing the seed for coming generations. Energy is extracted 

 from the environment chiefly by means of the gills and the digestive tract. 



In most teleosts the energy-containing yolk is small and the young larvae of 4 to 6 

 millimeters in length are already provided with small jaws, by means of which they can 

 feed upon still smaller organisms. These adaptive mechanisms of the larval stages tell us little 

 if anything about the adult stages of remote ancestors except in cases of "pedogenesis." 



In the late fry stages the body becomes deeper, especially in deep-bodied species, and 

 with it the occiput deepens vertically. Crests and ridges appear only in late fry and im- 

 mature stages as the muscles increase in strength (see below, p. 445). Although the 

 characteristic pattern of the adult skull is thus late in its appearance, it may nevertheless 

 be regarded as the result of the interaction of hereditary forces with the normal factors of 

 growth and environment. 



For information as to the evolution of the adult skull of any particular type, we must 

 therefore seek to understand both its adult functions and its developmental history; we 

 must compare it with less specialized skulls of its own group and assemble the available 

 palseontological evidence as to its derivation. 



As to the functions of the skull in the adult, we recall that the branchiocranium served 

 originally as a mechanism of jointed levers for operating the pumps and valves of the oxygen- 

 intake system. Secondarily, certain of its arches function as compound levers for raking in 

 food, or for breaking and crushing food, as well as for the support of the tunnel that leads to 

 the assimilative system. The neurocranium is primarily a thrust-box, anchorage or ful- 

 crum, fitted to receive and neutralize various thrusts and pulls from the body, from the 

 resisting medium, and from the active parts of the branchiocranium. At the same time it 

 groups and directs all the stresses and lines of force, above, below and around the central 

 nervous system, which is perfectly suspended from it without shock or strangulation of the 

 delicate nerve cables or brain parts. 



Both neurocranium and branchiocranium may be conceived as a complex system of 

 casts of osseous, cartilaginous and connective tissue material that has been excreted, so to 

 speak, in the spaces left between the more dynamic tissues. In general the skeletal tissues 

 are passive and plastic in relation to the nervous, muscular, nutritive and excretory tissues; 

 but skeletogenous tissue sometimes appears to assert its own growth force, as when it 

 produces thickenings and excrescences. 



The skull is thus a living palimpsest of many writings and from it we may in many 

 cases, after extended comparisons, decipher the story of its owner's way of life and even a 

 good part of his ancestral lineage. 



In our endeavor to decipher the habits of an individual fish from its skeleton we look 

 first at the jaws and teeth, the position. and direction of the mouth, the pharyngeal mill, etc., 



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