334 INSTINCTS OF THE MOLOTHRUS. [Chap. YHL 



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 some- 

 what 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 indi- 

 vidual 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 

 instinct-. 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 throw- 

 ing out the nestlings of the stranger. They either lay 

 their eggs in the nest thus appropriated, or oddly enough 

 build one for themselves on the top of it. They usually 

 sit on their own eggs and rear their own young; but 

 Mr. Hudson says it is probable that they are occasion- 

 ally parasitic, for he has seen the young of this species 



