12 Evolution and Distribution of Fishes 



tors, which In varying strength, direction, time and point 

 of application, affect each and every organism, so as to 

 bring about one of three possible results. These results 

 may be: (a) steady and progressive, or at times rather 

 sudden and marked (mutational) evolution, into types that 

 depart more and more from the parent forms; (b) an 

 evenly balanced or equilibrated organic relation to sur- 

 rounding factors, so that through a more or less pro- 

 longed period of time, an organism or group of hereditarily 

 derived organisms may remain apparently unchanged In 

 structure and function; (c) a steady or rapid maladjust- 

 ment to environal agents, so that devolution and degen- 

 eration, may ensue, succeeded it may be by death of the 

 organism or group of organisms involved. 



It follows from the above statements that the factors 

 of Pentamorphogeny which affect every organism, and the 

 results which ensue, either in evolution, equilibration, or 

 devolution of such organism, owe their inception to en- 

 vironal agents. So it is appropriate here to mention at least 

 some of the agents that have operated in the past either 

 to evolve, to continue, or to destroy the organisms specially 

 treated of in this volume. These environal agents are now 

 being more and more minutely studied under the term 

 ecology. And for extended consideration the reader is 

 referred to special books in this biological field. It is 

 however important to note that all of these agents represent 

 the action of some form of energy, whether of inorganic 

 or of organic origin. 



Such universal environal agents as heat (thermic 

 energy), light (lumlc energy), chemic compounds (chemic 

 energy), electricity (electric energy), geotropism (gravic 

 energy), in their varied exhibitions, all act either together 

 or more or less apart, so as to affect higher Invertebrates 

 and fishes. Thus it has been observed that widespread 

 destruction of great shoals of fishes may result, if In the 

 case of tropical species, the temperature rather suddenly 

 drops, and conversely the same result may ensue if, through 

 rise in temperature owing to submarine volcanic action, 

 oceanic waters become heated above a definite optimum 

 for fishes of temperate seas. 



