266 THE STRUCTURE AND LIFE OF BIRDS cHapP. 
more dependable. These two investigators found 
that in expenditure on flight muscle a big bird is 
more economical than a small one of the same family, 
a big Tern than a small Tern, a big Gull than a small 
Gull. Even when birds of different families are com- 
pared, the rule generally holds. The Eagle and other 
birds of strong flight have, for their size, lighter muscles 
than small birds. The heavy-flying Geese are an ex- 
ception. Incalculable factors complicate the problem, 
but, clearly, many big birds can save in muscle and 
devote more vital energy to other organs.! 
Helmholtz undertook to show that a big flying bird 
is an impossibility, since as the supporting power 
increases the weight increases more rapidly.2 This 
runs quite counter to the figures just quoted. Even 
if his calculation is right in the abstract, it ignores 
some important facts that ought to make us hesitate 
before we apply his conclusion to birds as they are. 
It ignores the fact that the work done by a large wing 
surpasses the work of a small wing by far more than 
the superiority in area. It disregards what, if not 
established fact, is yet not far from it, viz. that 
pterodactyls, with wings measuring 25 feet from tip 
to tip, were able to fly. It is now still further 
1 In three Terns, the comparative weights of which are repre- 
sented by the figures 53, 116,174, the total weights were 519, 
63%, 77%3; times the weight of the breast-muscles in the re- 
spective specimens. See Jahresbericht der Schlesischen Gesell- 
schaft fiir Vaterland-Cultur. Breslau, 1879. 
2 The calculation was as follows: If the linear dimensions 
increase as I : 4, then bulk and consequent weight’(and presum- 
ably strength) will increase as 1 : 43, that is, as 1 :64, but the 
sustaining force must increase as I : 43, that is, as 1 : 128, 
