PARENTAL CARE AMONG FRESH-WATER FISHES. 409 



gle instance in which the male is hirger tlian the female."' '^ Yet in 

 the very chaj^ter containing this assertion Darwin represents three 

 species of fishes selected by Giinther in which males are figured of 

 larger size than females; they are named Callionymus lyra (fig. 29), 

 Xiphophorus hcUerl (fig. 30), and Plecostomus harhatus (fig. 31). 

 These species, be it remarked, have the sexes trenchantly differen- 

 tiated, the males in two cases being marked by a superior brilliancy 

 of coloration and exuberance of fins and in the other {Plecostonms) by 

 a bristly armature of the head. The instances of increased size of 

 the male are, in fact, numerous; in almost all cases when the males 

 are decidedly differentiated from the females by brilliancy of color 

 or other secondary characters the rule is that they are larger than the 

 females. Like other rules, this may be subject to exceptions, but the 

 rule is based on extensive observations. A couple of figures made 



Male. Female. 



Figs. 2, 3. — Ancistrus ucciilcutalis. 



simply to illustrate sexual differences for Mr. C. Tate Regan's Mono- 

 graph of the Loricarioid Catfishes of America and reproduced here 

 exemplify the rule. Of a kindred species, Pseudancistrus harhatus,^ 

 the sexes are illustrated in Darwin's work. 



During the consideration of the social economy of these fishes the 

 question must often recur. How did the parental instinct manifested 

 originate? It was easy enough in olden time to give an ansAver 

 whi,ch would be regarded as all sufficient in those days ; it was a spe- 

 cific instinct implanted by an omnipotent creator in every case. In 

 these days of evolutionary belief, however, such an answer is equiva- 

 lent to no answer. The instinct must be regarded as a development 

 of an aptitude inherent in the fish itself. 



a Darwin, The Descent of Man, N. Y., 1881, p. 335. 



b Plecostomits Ixirlxitiis Gunther, Darwin; Anci-'<tnin harhatiix. Regan. 



SM 1905 30 



