A Theory of the Parasitic Hahit of the Cuckoo. 63 



would not the necessity have been even less long ago ? Food, 

 Macgillivray says, remains abundant, and tlie climate which 

 does not injure the young for two months longer could 

 liardly incommode the parents. The individual advantage 

 to the parent is certainly not very obvious so far as migra- 

 tion is concerned. Yet the advantage ouglit still to be 

 apparent, for one of our greatest modern Darwinians says 

 that natural selection must continually operate to conserve 

 and retain what it has previously gained. 



Is the advantage, then, for the offspring rather than for the 

 parent ? Did the original starters of the trick really have a 

 careful forethought of the best for their offspring in sending 

 them out to nurse ? Does a memory of the comfort of its 

 fostered youth remain with the adult as an impulse to do the 

 like for its young in turn ? It would be interesting to 

 inquire what form the memory takes in the preponderant 

 males. On the whole, does this not seem subtler than nature 

 dreams of ? 



But again, if the trick be simply a freak, without any 

 stated impulse behind it or near-at-hand advantage in front 

 of it, is not the common difficulty of the combination of 

 happy circumstances required to ensure incipient success 

 unusually great ? And does not the success of the plan 

 involve the presupposition of that monopolising tendency of 

 the young bird, which the theory gives no account of ? Then 

 the difficulty as to the inheritance of such a freak, especially 

 with preponderant males, is certainly appreciable. Such a 

 device must have something behind it, before it can be 

 transmitted. In short, except the direct individual gain 

 through saving of trouble and sacrifice, the advantages 

 hardly seem by themselves capable of justifying the elabora- 

 tion of the device into an instinct by natural selection. 



(c.) Before passing from objections, I may also urge the 

 increased difficulty raised when we consider the occurrence 

 of the habit. It is not generic, that is to say it is not 

 practised by all the species of Cuculus. It obtains in related 

 genera, however. Did it arise in some common progenitor ? 

 But it also occurs, in varied degrees of perfectness, in the 

 widely separated, starling-like cow-birds. There again, there- 



