GROWTH 'AND SECONDARY SEXUAL CHARACTERS. XCl 



to nature and withdrawing control is almost as ruinous as if arable lands 

 were to be left to be self-sown: while want of food may be occasioned 

 by too large a population of small and' inferior sorts which starve the 

 remainder. 



During cold months, also during the breeding' season, a cessation in 

 taking food may occur in some forms (page Ix), but during such periods as 

 they are on the feed they may obtain it from the bottom or soil orer which 

 the water flows or is placed, from mid- water as substances which are washed 

 down or subside from the surface, &c., or from the surface itself as flies or 

 vegetable food. 



Although the growth offish is often irregular, as . a general rule they do 

 not alter their form to any very great extent from what they are when, 

 young to what obtains during mature life; still the comparative increase of 

 certain parts to that of the entire animal may not continue the same. Thus, 

 the relative size of the eye to that of the head decreases, the armature about 

 the head may diminish or disappear, the form of the snout as seen in saw- 

 fishes may chaUge (plate Ixix) as well as. the character of. the fins (page ix). 

 While ■ iij . sonie pelagic forms Hemimetamorphosis may occur, or very 

 considerable alterations in -their growth and development which have not 

 infrequently occasioned misunderstanding as to the genus or 'even fatnily 

 to which the young individual belong?. More complete metamorphosis is 

 observable in the lampern (vol. ii, p. 362), while the deciduous external 

 gills of foetal sharks, rays (plate clxv), and a few other fish, may almost be 

 deemed to belong to this group. 



Secondary sexual characters likewise induce changes. Thus, although 

 the male salmon has a knob-like tubercle on the .lower jaw, the young has 

 none, but is similar to the female (page ,lvii). The skin may change, 

 becoming more rough in one sex, and spines appear most developed during 

 •the breeding-season, as in the rays (vol. ii, p. 329). The teeth Ukewise 

 may be different in adults of the two sexes. Other instances are alluded to 

 in the following pages. 



The size of some fish may increase more rapidly under certain conditions 

 than it does in others ; a few seeto to bear confinement easily, and grow 

 large when well fed in an aquarium. In the Southport Aquarium turbot 

 received, 3 inches across in size, became in two years 10 lb. each in weight, 

 and after two years, more they further augmented to 20 lb. Many forms 

 will live when in a contracted space of water without increasing in size. 

 While degeneration in size may be owing to local causes, as want of 

 sufficient nourishment, but be entirely distinct from degeneration in 

 structure or function. In short, growth may greatly depend on the food 

 which is obtainable, and which fish have usually to disperse and follow after. 



