I04 KINSHIP OF BIRDS, AS SHO WN 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 {Tympa- 

 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 

 Alcidce 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 oxxx Pygopodes 

 have come about in this way, nor that the theory accounts 

 for the jamming up of the " twenty-odd " vertebrae in the 

 tails of the successors of the Archceopteryx. 



The Pigeons, with usually horizontal bodies, lay very 

 ellipsoidal eggs, but the Sand Grouse (Pterodetes), 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 



