DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 2( 



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extremities and tail in the Flying Lemur (Plate XIII. fig. 18) and Bat (Piatt; XIII 

 fig. 15). Although no Lizard is at present known to fly, there can be little doubt tha 

 the extinct Pterodactyles (which, according to Professor Huxley, are intermediate be 

 tween the Lizards and Crocodiles) were possessed of this power. The 15at is interest 

 being the only mammal at present enjoying the privilege of flight*. It affords an ex- 

 treme example of modification for a special purpose, — its attenuated body, dwarfed pos- 

 terior and greatly elongated anterior extremities, with their enormous fingers and out- 

 spreading membrane, completely unfitting it for terrestrial progression. It is instruc- 

 tive as showing that flight may be attained, without the aid of hollow bones and air-saes, 

 by purely muscular efforts, and by the mere contraction and dilatation of a continuous 

 membrane. The structure and action of the Bat's wing are fully described at a subse- 

 quent page. 



PROGRESSION ON AND IN THE WATER. 



If we direct our attention to the water, we encounter a medium less dense than the 

 earth and considerably more dense than the air. As this element, in virtue of its 

 fluidity, yields readily to external pressure, it follows that a certain relation exists be- 

 lt and the shape, size, and weight of the animal progressing along or through it 



those animals making the greatest headway which are of the same specific gravity, or 



a little heavier, and are furnished with extensive surfaces which, by a dexterous tilting or 

 twisting (for the one implies the other), or by a sudden contraction and dilatation, they 

 can apply wholly or in part to obtain the maximum of resistance in the one direction, 

 and the minimum of displacement in the other. This arises from the fact, first pointed 

 out by Sir Isaac Newton, that bodies or animals moving in water and likewise in air 

 experience a sensible resistance, which is greater or less in proportion to the density and 

 tenacity of the fluid and the figure, superficies, and velocity of the animal. 



To obtain the degree of resistance and non-resistance necessary for progression m 

 water Nature, never°at fault, has devised some highly ingenious expedients,— the Syringo- 

 crade'animals advancing by alternately sucking up and ejecting the water in which they 

 are immersed-the Medusa by a rhythmical contraction and dilatation of then mushroom- 

 shaped disk-the Botifcra or wheel-animalcules by a vibratile action of their cilia, which, 

 . JvTdinc to the late Professor Quekett, twist upon their pedicles so as alternately to 



diminish the extent of surface presented to the water, as happens m the 

 feathering of an oar. A very similar plan is adopted by the Pteropoda, found in countless 

 multitudes in the northern seas, which, according to Eschricht, use the wing-like struc- 

 tures situated near the head after the manner of a double paddle, resembling in its 

 general features that at present in use among the Greenlanders. The characteristic 

 movement however, and that adopted in by far the greater number of instances, is that 



i ' ;„ +Via fioh (Tkte XII fi"\ 11). This, as my readers are aware, consists of 

 commonly seen m the nsn (liare^ix. u .x , > . 



a lashin- curvilinear, or flail-like movement of the broadly expanded tail, which oscillates 

 from side to side of the body, in some instances with immense speed and power. ™- 



The Vampire Bat of the island of Bonin, according to Dr. Buckland, can also swim ; and this authority was of 

 opinion that the Pterodactyle enjoyed similar advantages (Bug. Cycl. vol. iv. p. 495). 



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