INSTINCTS OF THE MOLOTHRUS 273 



this age is in a most helpless condition, Mr. Gould was for- 

 merly inclined to believe that the act of ejection was per- 

 formed by the foster-parents themselves. But he has now re- 

 ceived a trustworthy account of a young cuckoo which was 

 actually seen, whilst still blind and not able even to hold up 

 its own head, in the act of ejecting its foster-brothers. One 

 of these was replaced in the nest by the observer, and was 

 again thrown out. With respect to the means by which this 

 strange and odious instinct was acquired, if it were of great 

 importance 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 acquired, 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 towards 

 the acquisition of the proper instinct might have been mere 

 unintentional restlessness on the part of the young bird, when 

 somewhat advanced in age and strength ; the habit having 

 been afterwards improved, and transmitted to an earlier age. 

 I can see no more difficulty in this, than in the unhatched 

 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 remarked, 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 inherited at a corresponding or earlier age, — propo- 

 sitions which cannot be disputed, — then the instincts and 

 structure 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 interest- 

 ing gradation in the perfection of their instincts. The sexes 

 of Molothrus badius are stated by an excellent observer, Mr. 

 Hudson, sometimes to live promiscuously together in flocks, 

 and sometimes to pair. They either build a nest of their own, 

 or seize on one belonging to some other bird, occasionally 

 throwing out the nestlings of the stranger. They either lay 



