170 



SEXUAL DIMORPHISM 



the female. Darwin remarks that it is not provided with 

 muscles, and therefore cannot be used in locomotion, but it 

 must be admitted that it may be of great importance in 

 locomotion in offering resistance to the water when the body 

 and tail are swayed from side to side by the body muscles. 

 The movement of the body in swimming mechanically tends 

 to flatten the body in a vertical direction. If a round body 

 covered with a flexible membrane is moved from side to 

 side in water the membrane will be stretched in the vertic 

 median plane of the body by the resistance of the water. 



A 



Fig. 18. Molge cristata, the Crested Newt, a British species. A, male ; B, female. 

 From specimens in the British Museum of Natural History. 



this way the skin of the newt would be mechanically stretched 

 in the middle line, and growth following the mechanical 

 tension would produce the crest. The membranous fin in 

 the larva of a fish, or in the tadpole of the newt, doubtless 

 arises in a similar way, and the crest of the male adult newt 

 may be regarded to some extent as the retained membranous 

 median fin of the tadpole. But not entirely so, because the 

 crest disappears after the breeding season. It is therefore 

 extremely probable that the crest of the male in these 

 species of newt is due to the active movements of the male 

 in courting the female in the breeding season. 





