c 



CHAPTER XI 



INSTINCTS, HABITS, AND ADAPTATIONS 



HE Habits of Fishes. — The habits of fishes can hardly 

 be summarized in any simple mode of classification. 

 In the usual course of fish-life the egg is laid in the 

 early spring, in water shallower than that in which the parents 

 spend their lives. In most cases it is hatched as the water 

 grows warmer. The eggs of the members of the salmon and 

 cod families are, however, mostly hatched in cooling waters. 

 The young fish gathers with others of its species in little schools, 

 feeds on smaller fishes of other species or of its own, grows and 

 hanges until maturity, deposits its eggs, and the cycle of life 

 begins again, while the old fish ultimately dies or is devoured. 



Irritability of Animals. — All animals, of whatever degree of 

 organization, show in life the quality of irritability or response 

 to external stimulus. Contact with external things produces 

 some effect on each of them, and this effect is something more 

 than the mere mechanical eft'ect on the matter of which the 

 animal is composed. In the one-celled animals the functions 

 of response to external stimulus are not localized. The}- are 

 the property of any part of the protoplasm of the body. In the 

 higher or many-celled animals each of these functions is spe- 

 cialized and localized. A certain set of cells is set apart for each 

 function, and each organ or series of cells is released from all 

 functions save its own. 



Nerve-cells and Fibres. — In the development of the indi- 

 vidual animal certain cells from the primitive external layer 

 or ectoblast of the embryo are set apart to preside over the rela- 

 tions of the creature to its environment. These cells are highly 

 specialized, and while some of them are highly sensitive, others 

 are adapted for carrying or transmitting the stimuli received by 

 the sensitive cells, and still others have the function of receiv- 



