INSTINCT 361 



odious instinct was acquired, if it 'were of great impor- 

 tance for the young cuckoo, as is probably the case, to 

 receive as much food as possible soon after birth, I can 

 see no special difficulty in its having gradually ac- 

 quired, during successive generations, the blind desire, 

 the strength and structure necessary for the work of 

 ejection; for those young cuckoos which had such habits 

 and structure best developed would be the most securely 

 reared. The first step toward the acquisition of the 

 proper instinct might have been mere unintentional rest- 

 lessness on the part of the young bird, when somewhat 

 advanced in age and strength; the habit having been 

 afterward improved, and transmitted to an earlier age. 

 I can see no more difficulty in this than in the un- 

 hatched young of other birds acquiring the instinct to 

 break through their own shells; or than in young 

 snakes acquiring in their upper jaws, as Owen has re- 

 marked, a transitory sharp tooth for cutting through the 

 tough egg-shell. For if each part is liable to individual 

 variations at all ages, and the variations tend to be in- 

 herited at a corresponding or earlier age — propositions 

 which cannot be disputed — then the instincts and struct- 

 ure of the young could be slowly modified as surely as 

 those of the adult; and both cases must stand or fall 

 together with the whole theory of natural selection. 



Some species of Molothrus, a widely distinct genus of 

 American birds, allied to our starlings, have parasitic 

 habits like those of the cuckoo; and the species present 

 an interesting gradation in the perfection of their in- 

 stincts. The sexes of Molothrus badius are stated by 

 an excellent observer, Mr. Hudson, sometimes to live 



promiscuously together in flocks, and sometimes to pairo 



— Science — 16 



