» 124 NATURAL HISTORY OF WASPS. — 
sured by thousands of strokes in the minute, their 
endurance by miles and hours of flight. 
Interesting as this problem is in the thought that 
perhaps such abstract speculations may be some day 
turned to practical account, yet a physiologist can 
scarcely help dwelling on the fact that size seems to 
set limits to the denizens of the air, as it does to 
those which walk on the earth, and, with a wider 
margin however, to those whose bulk is to be sup- 
ported by the waters of the sea. If we hope ever to 
make the air available for purposes of transit we 
must look beyond animal substances for materials. 
Something more than muscle is wanted to supply 
the strength, something more than bone to bear 
the strain to be put on it. But steel and the sources 
of steam are too heavy for flight, and the vapour that 
will submit to be pent up in a bottle, like the Efreet 
in the Arabian Nights Tales, and to be practically 
applied to the feat of driving us safely through the 
air, has yet to be discovered. 
