104. KINSHIP OF BIRDS, AS SHOWN BY EGGS. 
group; and in some Partridges, on the other. In our Bob 
White the eggs are usually quite pyriform (as these sharply- 
pointed eggs are called). Here the clutch is large and the 
eggs often lie in two layers. It is possibly a convenience to 
have the pointed eggs of one layer project into the interstices 
of the other, and here also they lie closer together. When it 
comes to so small a bird covering as many as thirty-six eggs, 
economy of space is an item. The Prairie Chicken (Zympa- 
muchus americanus) lays also a fairly large clutch, but it is 
strikingly my experience that she does not hatch out nearly 
so large a percentage of her clutch as does the Partridge 
(Colinus virginianus), 
The Auk-forms—some of them—lay a single very pyri- 
form egg, and being broad-breasted have no need of this 
shape to make incubation easy. It is well known, however, 
that it is claimed that this shape is maintained to prevent 
their egg from rolling off the bare rock upon which it is 
sometimes deposited. It is rather remarkable also that such 
Alcide as nest thus outside of burrows have the more un- 
equal-ended eggs, as if designed to roll around in a circle, 
Recalling the fact mentioned that the upright postures 
may tend to make the lower end of the egg larger by gravity, 
we can see how a double selection action might come in 
here. Thus the most upright bird laid the most pear-shaped 
egg; the most pyriform egg is the least liable to roll off and 
be broken, and a more upright-sitting and pyriform-laying 
strain would thus come about, till the poor bird would 
almost stand on the tail. I do not assert that our Pygopodes 
have come about in this way, nor that the theory accounts 
for the jamming up of the “twenty-odd ” vertebra in the 
tails of the successors of the Archaeopteryx. 
The Pigeons, with usually horizontal bodies, lay very 
ellipsoidal eggs, but the Sand Grouse (Prterocletes), which are 
the connecting link between these and the fowl-forms, lay 
an extravagantly elongate, equal-ended egg, and are terres- 
trial and Plover-like in nesting, in the number of eggs, and 
