PASSIVE AND ACTIVE YOUNG 121 



Cormorants, and Parrots, to say nothing of the 

 numerous tribe of Passerines, generally build above 

 ground on bushes, trees, or rocks ; and such will 

 naturally enough have helpless young ; those which 

 have the foot only formed for running or swimming, 

 and the hind-toe very small or absent, are naturally 

 less apt to perch, build on the ground as a rule, and 

 have young which are active runners or swimmers. 



It thus becomes possible in most cases to predict 

 the probable nature of a previously unknown bird's 

 young by the structure of the adult's foot ; but 

 such detective-story methods in natural history are 

 to be employed with great caution, and in the 

 present case, as might be expected, exceptions 

 occur. To take the flightless birds, for instance, the 

 rule works out all right with the flightless runners, 

 from Ostrich to Apteryx, all of which have non- 

 grasping feet with the hind-toe absent in all but 

 the last ; but it fails when we turn to the Penguins, 

 which have a dwarfed useless hind- toe, even situated 

 on the inner side of the foot, and yet have young 

 which are helpless nestlings, in spite of the labour 

 thus entailed on the flightless parents in trudging to 

 and from the sea to get supplies for them, a labour 

 in which the male is as earnest as the hen. 



Among the flying birds, too, we find that in the 

 Petrels, which are by no means perchers, having 

 the hind toe reduced to a nail, and breed more 

 often in holes in the ground than anywhere else, 

 that the young are helpless and fed with an oily 

 substance disgorged into their mouths by the parents. 



