CHAPTER VI 



EVOLUTION OF BODY FORM IN THE FISHES AND 



AMPHIBIANS 



Rapid evolution in a relatively constant environment. Mechanism of motion, 

 of offense, and defense. Early armored fishes. Primordial sharks. Rise 

 of existing groups of fishes. Form evolution of the amphibians. Maxi- 

 mum radiation and extinction. 



A SIGNIFICANT law of fish evolution is that in a practically 

 unchanging environment, that of salt and fresh water, which is 

 relatively constant both as to temperature and chemical con- 

 stitution as compared with the variations of the terrestrial 

 environment, it is steadily progressive and reaches the great- 

 est extremes of form and of function. This indicates that a 

 changing physicochemical environment, although important, is 

 not an essential cause of the evolution of form. The same 

 law holds true in the case of the marine invertebrates (p. 137), 

 as observed by Perrin Smith. A second principle of signifi- 

 cance is that even the lowliest fishes establish the chief glandu- 

 lar and other organs of action, reaction, and interaction which 

 we observe in the higher types of the vertebrates. Especially 

 the glands of internal secretion (p. 74), the centres of inter- 

 action and coordination, are fully developed. 



Mechanism of Motion, of Offense, and Defense 



Ordovician time, the early Palaeozoic Epoch next above the 

 Cambrian, is the period of the first vertebrates known, namely, 

 the fossil remains of fish dermal defenses found near Canon 

 City, Col., as announced by Walcott in 1891, and subse- 

 quently discovered in the region of the present Bighorn 



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