172 



SEXUAL DIMORPHISM 



generally in the direction of mechanical tension, the experi- 

 ment strongly supports the conclusion that in the majority 

 of cases no other explanation of the evolution of dermal webe 

 in animals is required than the tension produced by the 

 resistance of the surrounding medium, whether air or water, 

 but especially water. Thus we learn why aquatic birds and 

 aquatic mammals have webbed feet. Probably the dermal 

 membranes of flying mammals and reptiles owe their origin 

 to a similar cause. The membranous vertical fins of larval 

 fishes, of tadpoles, and of newts are thus explained directly 

 as growths resulting from the tension caused by the resist- 

 ance of the water, when the posterior part of the body is 

 moved from side to side. 



There are, however, various details to be considered in 

 applying the above principle to the explanation of the dorsal 

 crest in male newts. One question is, why is there not a 

 corresponding ventral crest on the body, as there is to some 

 extent on the tail ? Another is, why do some species possess 

 the crest on the body and others not ? I believe that all 

 such questions could be answered by a sufficiently minute 

 study of the movements of the males in courtship in the 

 different species. These movements have been described 

 carefully by various observers in certain species. One of 

 the best of such descriptions is that of Dr. Rusconi, 1 

 published at Milan in 1821. In reference to a species 

 which he names Salamandra platycauda, and which is 

 almost identical with our own Molge cristata, he writes 

 that the female, when tired of fleeing, rests motionless on the 

 ground, that the male then places himself with his side 

 against her snout, bends his body, supports himself on his 

 fore-feet alone, and, shaking his tail, commences to thrash 

 the water. After having agitated his tail and body thus 

 for half a minute, he strikes the side of the female with 



1 Amours cles salamandres aquatiques. 



