INSTINGTS OF THE MOLOTHRUS. 253 



In the case of the European cuckoo, the offspring of the 

 foster-parents are commonly ejected from the nest within 

 three days after the cuckoo is hatched; and as the latter 

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

 formerly inclined to believe that the act of ejection was 

 performed by the foster-parents themselves. But he has 

 now received a trustworthy account of a young cuckoo 

 which was actually seen, while 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 ac- 

 quired, 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, 1 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 cuckoos whiph 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 restlessness 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 diffi- 

 culty 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 — proposi- 

 tions 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 

 interesting 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 



