42 Instincts, Habits, and Adaptations 
trolled by the impulse to reproduce. Some pelagic fishes, espe- 
cially flying fishes and fishes allied to the mackerel, swim long 
distances to a region favorable for a deposition of spawn. Some 
species are known only in the waters they make their breeding 
homes, the individuals being scattered through the wide seas at 
other times. Many fresh-water fishes, as trout, suckers, etc., for- 
sake the large streams in the spring, ascending the small brooks 
Fic. 28—Jaws of Nemichthys avocetta Jordan and Gilbert. 
where they can rear their young in greater safety. Still others, 
known as anadromous fishes, feed and mature in the sea, but 
ascend the rivers as the impulse of reproduction grows strong. 
An account of these is given in a subsequent paragraph. 
Variability of Instincts.— When we study instincts of ani- 
mals with care and in detail, we find that their regularity is 
much less than has been supposed. There is as much variation 
in regard to instinct among individuals as there is with regard 
to other characters of the species. Some power of choice is 
found in almost every operation of instinct. Even the most 
machine-like instinct shows some degree of adaptability to new 
conditions. On the other hand, in no animal does reason show 
entire freedom from automatism or reflex action. “The funda- 
mental identity of instinct with intelligence,’ says Dr. Charles 
O. Whitman, “is shown in their dependence upon the same 
structural mechanism (the brain and nerves) and in their re- 
sponsive adaptability.” 
Adaptation to Environment.—In general food-securing struc- 
tures are connected with the mouth, or, as in the anglers, are 
hung as lures above it; spines of offense and defense, electric 
organs, poison-glands, and the like are used in self-protection; 
the bright nuptial colors and adornments of the breeding sea- 
son are doubtfully classed as useful in rivalry; the egg-sacs, 
nests, and other structures or habits may serve to defend the 
young, while skinny flaps, sand or weed-like markings, and 
