120 BIOLOGICAL LECTURES. 



belligerents touches one of the neutrals in the aquarium, it 

 frequently gives it a punch, but does not follow it up, and the 

 unoffending fellow makes haste to get out of the road, the 

 smaller ones doing this most quickly. If after an interval of 

 a few seconds a belligerent meets a neutral, they quietly pass 

 each other without paying any further attention, whereas, if 

 the two belligerents meet again, there is an immediate response. 

 Whether they recognize each other by touch or by their mutual 

 excitability I do not know. At one time, in another aquarium, 

 I saw one belligerent capture the other by the pectorals. 

 After holding on for a short time it let go and all differences 

 were forgotten. 



The thrust is delivered by a single vigorous flip of the tail 

 and caudal to one side. 



These fights were frequently noticed, and, as far as deter- 

 mined, always occurred between males. 



The absence of secondary sexual differences in the cave 

 fishes is a forcible argument in favor of sexual selection as the 

 factor producing high coloration in the males. The absence 

 of secondary sexual differences in caves opposes the idea of 

 Geddes and Thomson that the differences are the external 

 expression of maleness and femaleness. 



Attempts at acclimating Amblyopsis in outside waters have 

 largely failed. A few were placed in Turkey Lake, Indiana. 

 They were surrounded by a fine wire net to keep off other 

 fishes. They died in a few days as the result of attacks of 

 leeches, Saprolegnia, or fish mould, and from unknown causes. 

 Others were kept in an elongated box sunk into the ground 

 where fresh spring water flowed through it constantly. Sapro- 

 legnia sooner or later destroyed all of them. They live longest 

 in quiet aquaria where the water is rarely changed. The young 

 I have secured died, with one exception, within a few weeks. 

 The difficulty of rearing the young is not at all insurmountable. 

 They eat readily. Their aquaria must be kept free from green 

 plants and have a layer of fine mud with a few decaying leaves 

 in the bottom. They will feed on minute crustaceans and 

 other micro-organisms. When they have reached a sufficient 

 size, Asellus are greedily devoured. Fish mould is the bane of 



