i6 THE COURTSHIP OF ANIMALS 



by certain of the ductless glands " ; that is to say, 

 of glands having no apparent connection with their 

 surrounding tissues. We owe much of our knowledge of 

 this subject to Professor Starling, who has called these 

 secretions " Hormones." 



Darwin knew that the essential sexual glands, the testes 

 and the ovaries, in some mysterious way controlled, in 

 a large degree, the development of these " hall-marks " 

 of sex, for it was known in his time that castrated stags 

 failed to produce antlers, and that hen pheasants, for 

 example, in extreme old age, or when the ovaries were 

 damaged by disease or injury, at once assumed the 

 plumage of the cock ; but the part played by these 

 ductless glands was quite unsuspected. They are the 

 Thyroid, and the Thymus glands, which are attached 

 to the outer walls of the trachea or windpipe. The 

 Pituitary body, which forms part of the brain, and the 

 Suprarenal bodies, attached to the kidneys. It would 

 be foreign to the purpose of these pages to enter into the 

 functions of these glands ; suffice it to say, that the 

 juices formed therein are taken up by the blood, and 

 distributed over the system. Their action is only very 

 imperfectly understood. We know that any derange- 

 ment in their efficiency results in disease, and that they 

 play a very important part in the reproductive system, 

 as will become abundantly evident in the course of 

 these pages. Much hitherto attributed to the action of 

 " Sexual Selection " alone, it is now evident is largely due 

 to their action. 



The all-sufficiency of the " Sexual Selection " theory 

 to account for the development of armature, such as 

 horns, antlers, and the huge spine-like outgrowths 

 which form so conspicuous a feature of many of the 



