919 THE STORY OF THE BIRDS. 
hoactzin may be a living twig from the old stem that 
started the fowl-pigeon-plover-rail clusters; and an- 
other twig, sprouting near it, ay have grown away 
on up to the cuckoos. 
To such as will put a bit of study on matters of 
this sort, the story of the birds has various interesting 
readings. 
The broken, irregular, single line upon the diagram 
divides the precocial from the altricial birds. No. 7 
and some forms between 8 and 12 have naked and 
rather helpless nestlings. Except these and the pi- 
geons, all birds above the line E have downy nes- 
tlings, all below it (except the lyre bird) have com- 
paratively naked young. 
The double broken line touching the edge of the 
ostrich forms, and passing through the rail-plover- 
crane-heron forms—leaving a few of them dry-land- 
ward—cuts off to themselves the aquatic birds. It 
may be noted how nearly it corresponds to the other 
line, as if dense down had been developed at the 
promptings of the aquatic habit. 
