50 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



omy, but our knowledge of them is as yet inadequate. The importance 

 of these relations in the geographical distribution of vertebrates has 

 not been sufficiently recognized. It must be admitted that regulative 

 adaptation to special conditions may take place within the same 

 species. Thus cattle, with their numerous skin glands, their saliva- 

 dripping mouths, and their excrement, are great expenders of water, 

 and as a consequence most of them live in humid regions, and many 

 forms, such as water buffalos, are swamp dwellers. This group of ani- 

 mals is likely to be absent in steppe regions, though the bison of 

 North America formed an exception. Domestic cattle, however, have 

 become adapted to going without water for days at a time in the arid 

 peninsula of Lower California. 37 



The physical character of the air as a surrounding medium condi- 

 tions certain peculiarities of structure in terrestrial animals, and cer- 

 tain characters which were excluded by the nature of the aquatic 

 habitat become possible. 38 On account of its low density, the air does 

 not assist in supporting the body as the water does. This necessitates 

 a general stiffening of supporting structures. Forms with a gelatinous 

 body are excluded from terrestrial life, except the Mycetozoa for 

 which the decaying wood in which they live furnishes support. The 

 soft-bodied terrestrial forms are compelled to rest the whole extent 

 of their bodies on the earth, but these too are firmer in structure than 

 their aquatic relatives, as appears in the land planarians, earthworms, 

 and snails. A greater degree of independent motion is possible only 

 when an internal or external skeleton is well developed. 



Raised from the earth by means of stiffened limbs, the body offers 

 much less friction when in motion, and at the same time the air offers 

 less resistance than water. On the other hand, the loading of the limbs 

 limits the size of the body, since in consequence of the laws of statics, 

 doubling the size of the body will require more than a twofold in- 

 crease in the strength of the supports. Hence the air-breathing, skele- 

 ton-bearing forms of mollusks, arthropods, and vertebrates fall behind 

 their water-breathing relatives in maximum size. Even the largest 

 land snail of the genus Achatina does not reach the measurements of 

 the marine Tritonium. The giant insects and arachnids such as the 

 Hercules beetle {Dynastes hercules; 15 cm. long) and some grasshop- 

 pers (e.g., Palophus, 25-30 cm. long) are much smaller than the lobster, 

 the rock lobster, Palinurus, or the giant crab Kaempfferia kaempfferi. 

 Compared with the right whale of 29 meters length and a weight of 

 147,000 kg., the elephant, 3.5 meters long and 4000 kg. in weight, is a 

 dwarf, and even the weight of the giant extinct saurian Brontosaurus 

 20 meters long, which is estimated at 38,000 kg., is only one-fourth as 



