102 MEANS OF EXPRESSION Chap. IV. 



incidental, may be compared with the profuse sweating 

 from an agony of pain or terror. Nevertheless, it is re- 

 markable how slight an excitement often suffices to cause 

 the hair to become erect; as when two dogs pretend to 

 fight together in play. "We have, also, seen in a large 

 number of animals, belonging to widely distinct classes, 

 that the erection of the hair or feathers is almost always 

 accompanied by various voluntary movements — by 

 threatening gestures, opening the mouth, uncovering the 

 teeth, spreading out of the wings and tail by birds, and 

 by the utterance of harsh sounds; and the purpose of 

 these voluntary movements is unmistakable. Therefore 

 it seems hardly credible that the co-ordinated erection of 

 the dermal appendages, by which the animal is made to 

 appear larger and more terrible to its enemies or rivals, 

 should be altogether an incidental and purposeless result 

 of the disturbance of the sensorium. This seems almost 

 as incredible as that the erection by the hedgehog of its 

 spines, or of the quills by the porcupine, or of the orna- 

 mental plumes by many birds during their courtship, 

 should all be purposeless actions. 



We here encounter a great difficulty. How can the 

 contraction of the unstriped and involuntary arrectores 

 pili have been co-ordinated with that of various volun- 

 tary muscles for the same special purpose? If we could 

 believe that the arrectores primordially had been volun- 

 tary muscles, and had since lost their stripes and become 

 involuntary, the case would be comparatively simple. 

 I am not, however, aware that there is any evidence in 

 favour of this view; although the reversed transition 

 would not have presented any great difficulty, as the 

 voluntary muscles are in an unstriped condition in the 

 embryos of the higher animals, and in the larvae of some 

 crustaceans. Moreover in the deeper layers of the skin 

 of adult birds, the muscular network is, according to 



