306 THE AMERICAN NATURALIST [Vol. LIV 



ships and a strengthening of the new, until in the end one 

 may supplant the other. 



On a hypothetical conception of this kind it may be 

 understood that the continued production of sternal and 

 pubic callosities, generation after generation, has intro- 

 duced such fixed and intimate inter-relationships of the 

 structural parts concerned that in the end they come to 

 replace the old inter-relationships altogether and with 

 them the non-callous condition. The callosities are 

 formed antecedent to and apart from the primary stimuli. 

 Their appearance becomes accelerated, as it were, and 

 they arise even before the chick is hatched and the orig- 

 inal stimuli can be effective. They are not new charac- 

 ters which have come in, but are new as regards the onto- 

 genetic time at which they appear. 



The possibility of responses occurring without the 

 original normal stimuli may be illustrated from certain 

 of the instinctive sexual activities of the ostrich. At the 

 breeding season the cock bird performs the sexual dis- 

 play known as " rolling." He crouches on the ground 

 and with wings outspread rolls from side to side, his long 

 neck and head also taking part, the latter striking vigor- 

 ously against each side of the body alternately. Also as 

 he approaches sexual ripeness he begins to "bromm," 

 the B#nnd having often been compared with the roar of a 

 lion. The mouth being closed lie inflates the esophagus 

 until the neck as a whole becomes two or three times its 

 usual thickness and then forcibly expels the air through ' 

 the nasal passages, producing a booming noise of great 

 carrying power, consisting of two short notes and a long 

 one, the sequence being repeated from one to six or seven 

 times, and serving as a guide to the farmer as to the state 

 of sexual ripeness of the bird. Again, during actual 

 pairing, the cock mounts upon the back of the crouching 

 hen with his right foot upon her back and the left upon 

 the ground, and sways the front part of the body and 

 neck to and fro as the act is consummated. 



The above are three distinctive actions on the part of 

 the cock ostrich which are usually performed only at 

 sexual maturity, and may be deemed to be responses asso- 



