72 THE POPULAR SCIENCE MONTHLY. 



misconception. There is no reason to believe that our mnscnlar 

 energy is any whit less than that of insects — that, taking a bundle 

 of muscular fibers of equal cross-section, the contractile power is 

 any less in our case than in theirs. The explanation is easily 

 found in the principle above stated. These apparently wonderful 

 feats of insects are simply the result of their small size. Weight 

 fZecreases as well as increases as the volume, i. e., as the ciihe, while 

 strength of muscle only as the cross-section, i. e., as the square of 

 the diameter. Therefore, with decreasing size weight decreases 

 far more rapidly than strength, and therefore the ratio of strength 

 to size mcreases ; and therefore, finally, less and less energy is 

 consumed in support, and more and more is left over for motion. 

 If any one desires to pursue this subject further, he will find it 

 fully treated in a previous number (April, 1883) of " The Popular 

 Science Monthly,'' in an article by Delboeuf on " Dwarfs and 

 Giants." 



3. Application to Natural Flying. — Now, this same prin- 

 ciple of limit applies with greatly increased force to flying. There 

 is a limit of size and weight of a flying animal ; and on account of 

 the prodigious energy required for aerial locomotion iluit limit is 

 very low, not much more than fifty pounds, certainly much less than 

 one hundred pounds. The largest flying-birds, such as the bustard, 

 the turkey-cock, and the condor, rise with difficulty. They are 

 evidently near the limit. There are, indeed, birds which are much 

 larger, such as the ostrich, the emu, and especially the extinct 

 Dinornis and Epiornis, but these are all flightless. They do not 

 fly, not because their wings are aborted, but, on the contrary, their 

 wings became aborted because they did not fly, and they did not 

 fly because they had grown too large. Nature could not make them 

 fliers, and therefore did not try. Or rather, it might perhaps be 

 said, she tried her best and failed. Their wings became aborted 

 because their size had passed the limit of possibility of flight. I 

 imagine that the history of their evolution was briefly something 

 as follows : They sprang originally from large birds of heavy 

 flight ; but being somewhat isolated from severe competition, on 

 islands with abundant food, natural selection took the direction 

 of size and strength for victory in contest, rather than swift- 

 ness of flight for escape from danger. They quickly passed the 

 limit of size for flight, and their wings becoming useless were 

 aborted. 



4. Relation of Rising to Propulsion in Flight. — There is 

 another principle involved in flight which must now be stated. 

 There are two things to be considered, viz., rising and propulsion. 

 We have already shown that the ratio of weight to strength, and 

 therefore the difficulty of rising, i?icreases as the size or weight. 

 We now add that the resistance of the air to motion through it. 



