Preliminary Definitions and Illustrations. 9 



exist between structure and habit, it must be noted that 

 there are many instinctive activities that no examination 

 of structure, with our present means and appliances, would 

 enable us to foretell. In the tributaries of the Severn 

 there is a spring migration of little eels, or elvers. Thou- 

 sands may be seen swimming up stream, and even 

 surmounting such obstacles as small waterfalls and mill- 

 sluices by wriggling up the conferva-clad surface. But 

 there is nothing in their structure to give a hint of this 

 migratory instinct. If an ant-lion larva were given to a 

 naturalist ignorant of its habits, he would no doubt be 

 able to say that it led a predatory life. He would not infer 

 from its structure that it made a conical pit in the sand 

 to entrap unwary ants and other insects, and, as it lay 

 almost concealed at the apex of the cone, scattered a 

 shower of sand-grains, by which its victim was brought down 

 into its very jaws. No examination of the structure of a 

 limpet would enable us to foretell that it would wander in 

 search of food within a radius of about a yard from its 

 scar, performing its peregrination when the rocks were 

 wetted by the tide, but returning home to its own particular 

 scar on the rock ere the sun and air had baked and dried 

 the surface, and ere the returning tide had deeply sub- 

 merged its home.* Very many such cases might be given 

 of activities to which the study of structure alone would 

 afford no clue. 



The fact that the same organ often subserves more 

 than one purpose may sometimes lead one astray in 

 attempting to infer habit from structure. In Trinidad 

 there occurs a climbing picarian bird, the red-tailed 

 jacamar (Galbula ruficauda). It has a long, sharp, pointed 

 bill, and no one from a study of its structure would be 

 likely to foretell that it subsists on flies, which it catches 



* See Nature, vol. xxxi. p. 200; and vol. li. pp. 127 and 511. 



