XVI THE NERVOUS SYSTEM 307 



held for a long time, the tension or cramp of the muscles 

 increasing, the longer the bodies were held. Goltz has sug- 

 gested that there is something that emanates from the body 

 of the female that acts as aspecial excitant to the male, but 

 there is not sufficient evidence for adopting this view, and 

 the only reason for accepting it is that other explanations of 

 sex recognition do not suffice. 



There is so much variation in the behavior of different 

 individuals, and so many factors that may determine whether 

 the clasping reflex may or may not persist in particular cases, 

 that it is hazardous to draw conclusions from single experi- 

 ments. Differences in tactile stimulation may play a role in 

 sex recognition, and differences in the movements of the two 

 sexes may be another factor. The experiments of Goltz, 

 although very instructive and suggestive, have not succeeded 

 in showing sex recognition to be the result of any one cause, 

 and it is not improbable that it may depend upon several 

 factors, each of which is not entirely indispensable. 



Sex discrimination in frogs is not very precise. Males 

 are sometimes held by other males for a long time, and 

 copulation is not infrequently known to occur between the 

 males of different species. Spallanzani records a male toad 

 that was carrying around an individual of his own sex that 

 had died some days previously and was in an advanced state 

 of decay. Whether or not a male will clasp an object other 

 than the female depends upon the length of time he has 

 been separated from the sexual embrace. If a male is 

 recently torn from a female, he is very sluggish, and will 

 clasp the fingers or almost any object that is presented to 

 him. If left for a time, he becomes more active and shows 

 little tendency to clasp all sorts of objects. Male frogs 

 which at first would be seized are now usually rejected, or 

 held but for a short time ; but if a female is presented, she 

 is seized with eagerness. 



