No. 3.] SEXUAL SELECTION IN SPIDERS. 135 



far. The point to be proved is thuL there is a concomitant 

 variation between color, plumes and other sexual ornaments 

 and the functional develojiment of the surfaces upon which 

 they are found. We are investigating a question of cause and 

 effect, and we are to prove that in places where there is great 

 functional activity, the cause, there is found sexual ornamenta- 

 tion, the effect. Mr. Wallace's way of stating the relation is a 

 little different. He gives instances of the development of 

 plumes from certain well-nourished parts, showing that in some 

 cases these ornaments arise from the most powerful muscles of 

 the body, and near the spot at which the activity of the muscle 

 is at its maximum ; and he implies that sexual ornaments 

 always arise from such muscular and well-nourished parts. 

 We even understand him to believe that there is a quantitative 

 proportion between the muscular development and the orna- 

 mentation. To quote his own words : 



" The view that color has arisen over surfaces where mus- 

 cular and nervous development is considerable, and the fact 

 that it appears especially upon the accessory or highly developed 

 plumes, leads us to inquire whether the same cause has not 

 primarily determined the development of these plumes. The 

 immense tuft of golden plumage in the^best. known birds of 

 paradise (Paradisea apoda and P. minor) springs from a very 

 small area on the side of the breast. Mr. Frank E. Beddard, 

 who lias kindly examined a specimen for me, says that 'this 

 area lies upon the pectoral muscles, and near to the point where 

 the fibres of the muscle converge towards their attachment to 

 the humerus. The plumes arise, tiierefore, close to the most 

 powerful muscle of the body, and near to where the activities 

 of that muscle would be at a maximum. Furthermore, the 

 area of attachment of the plumes is just above the point where 

 the arteries and nerves for the supply of the pectoral muscles, 

 and neighboring regions, leave the interior of the body. The 

 area of attachment of the plume is, also, as you say in your 

 letter, just above the junction of the caracoid and sternum.' 

 Ornamental plumes of considerable size rise from the same part 

 in many other species of paradise birds, sometimes extending 



