180 ANIMAL STUDIES 



become mature they emigrate in great companies, and make 

 their way hundreds, perhaps thousands, of miles to the 

 rivers in which they spent their youth. Up these streams 

 they rush in crowds, leaping over waterfalls and rapids, 

 and, dashed and battered on the rocks, many, and in some 

 species all, die from injuries or exhaustion after the breed- 

 ing season is passed. The eggs, like those of the chubs, 

 suckers, sunfishes, and catfishes, are usually buried in shal- 

 low holes in the sand, and the males of most fishes keep a 

 faithful watch over the young until they are able to live in 

 safety. In some of the sticklebacks and in several marine 

 species elaborate nests are composed of grass or seaweeds ; 

 some of the catfishes carry the eggs until they hatch in 

 their mouths or else in folds of spongy skin on the under 

 side of the body ; in the pipefishes and sea-horses a slender 

 sac along the lower surface of the male acts as a brood- 

 pouch, in which the female places the eggs to remain until 

 developed ; and some fishes, such as the surf-fishes and a 

 number of the sharks, bring forth their young alive. On 

 the other hand, the young of many of the herrings, salmon, 

 cod, perch, and numerous other fishes are abandoned at 

 their birth, and fall a prey to many animals, even the par- 

 ents often devouring their own eggs. 



In the former cases, where the young are protected, only 

 a relatively few eggs are produced : where they are aban- 

 doned the female often lays many millions. In every case 

 the number of eggs is in direct relation to the chances the 

 young have of reaching maturity, a few out of each brood 

 surviving to perpetuate the race. 



171. Development and past history. The eggs of the 

 higher bony fishes are usually small (one-tenth to one-third 

 of an inch in diameter), and the young when they hatch 

 are accordingly little ; in the sharks the eggs are larger, 

 the size of a hen's egg or even larger, and the young when 

 born are relatively large and powerful. These differences, 

 however, do not greatly affect the early development, for 



