72 



THE A M ERICA N NA 1 URA LIST. [Vol. XXXVII. 



ZOOLOGY. 



The Habits of Fishes. 1 — The author contends that : Physiologi- 

 cally, fresh water (and probably all) fishes fall into two groups, — 

 those which spawn in warming water and those which spawn in cool- 

 ing water, and the cause of spawning is the temperature trend in one 

 direction ; structurally similar forms tend strongly to sustain similar 

 relations to the temperature curve, i.e., to spawn either all on its 

 ascending, or all on its descending limb ; in at least some cases 

 apparent exceptions can be harmonized with the law ; for a given 

 species the temperature relations which determine its migrations, and 

 probably also its geographical distribution, are the same as those 

 which determine its spawning. These facts demonstrate the presence 

 of a temperature-responsive nerve-mechanism, which is a character of 

 prime importance, entitled to at least superfamily rank ; its existence 

 explains why with in-cooling spawning, 2 is (and must be) associated 

 to-cooler migration 2 and boreal distribution (and with in-warming 

 spawning, to-warmer migration and austral distribution) ; by a work- 

 ing backwards from the time of most successful hatching, the time 

 of spawning has been determined via natural selection ; that time 

 so fixed, by a further working backward natural selection has deter- 

 mined the time of precedent migration ; there are, de facto, beach 

 spawners ; in type of egg the beach spawners agree with the fresh 

 water, and differ from the pelagic, forms, and this difference explains 

 why species of pelagic genera are so rare in fresh water, and beach 

 spawners are now uncommon, they having mostly become ana- 

 dromes ; for having attained to a seek-the-beach impulse, the con- 

 ditions on the beach were such that, natural selection not opposing, 

 the beach spawners must, through the mere continued action of the 

 temperature-responsive mechanism, have been led, step by step, into 

 the forming streams of a rising continent ; and in the streams the 

 necessary accessory instincts have been evolved, all in accordance 

 with accepted biological principles. The most important generaliza- 

 tions are : Dynamically, fishes fall into two great groups according as 

 they are stimulated to migrate geographically, to migrate for spawn- 

 ing, and to spawn, by warming water, or by cooling water ; and this 

 dynamic factor necessarily involves a northern limit to the range of 



i Gurley, R. R. Atner. Journ. Psychol., vol. xiii (July, ,902), pp. 408-425. 

 teJixlm^xm™™^ 8RaWning in CO ° ling Water; to - cooler Ration, migra- 



