i8 Evolution and Distribution of Fishes 



In some cases even it seems to be undoubtedly true that 

 what at one time aided powerfully in defense might become 

 by degrees a burden, and ultimately an impediment to 

 life-activity and survival. Thus the heavily armored and 

 unwieldy Cephalaspids, Pterichthyids, Dinichthids, and 

 Titanichthids of Devonian age (pp. 129) largely survived 

 up to a certain stage, owing to steadily increased arma- 

 ment. But the very weight of this caused them to become 

 bottom feeders, which gradually were blotted out along- 

 side more active and predaceous elasmobranchs and ganoids. 



Natural selection then, or selective survival, is largely 

 a secondary and passive agency in the great pentamorpho- 

 genic act that organisms are molded by, during their entire 

 life period. 



V. REPRODUCTION. 



Our knowledge of this phase of pentamorphogeny — 

 so far as it concerns animals — is still largely in the making. 

 The writer has given a few reasons for regarding it as an 

 important factor for plants. In his "Descent of Man," 

 Darwin advanced arguments in favor of "Sexual Selec- 

 tion," but the potency of this has been very variously 

 viewed by later authors. The fact has long been known 

 that amongst fishes a sexual dimorphism of marked kind 

 may exist. This also becomes associated with structural 

 differences that must tend to upset the specific equilibrium 

 of the bisexual individuals. Thus the claspers and dorsal 

 spines on the males of various Rays, the bright coloration 

 of the males of some Teleosts during breeding season, 

 the at times smaller size of the males as compared with 

 the females as in the Bowfin {Amia calva) , the enlarge- 

 ment of the oval fin in the male of Polypterus, and not 

 least the nest-building habits of the Stickleback and of 

 various African fishes, give rise to structural differences 

 that must more or less tend to variation in the type. But 

 until wider studies are forthcoming it would be impossible 

 to estimate the potency of these details as evolutionary 

 factors. 



The capacity for artificial hybridization between species 

 of a genus has been definitely established for Salmo by Day 



