72 The Substratum 



serve as ecologically significant substrata. As rock breaks down it 

 produces areas of stony ground, gravel, and sand; and with the ad- 

 mixture of organic matter soil is formed from the parent rock sub- 

 stance. Each of these materials has its influence in controlling the 

 growth and distribution of plants and animals. As in the aquatic en- 

 vironment the land substrata provide purchase, shelter, attachment, 

 and nourishment in varying degree according to circumstances. 



Land Surfaces and Animals. Physical differences in the land 

 surfaces are correlated with special adaptations of animals inhabiting 

 them. On rocky terrains and in regions with hard, open ground the 

 running speed of animals is improved by the possession of small re- 

 sistant feet usually with a reduced number of toes, as in the deer, ante- 

 lope, and ostrich. Animals living in areas of soft sand, marsh, or 

 snow are characterized by spread-out feet, like those of the camel, or 

 of toes that present a large surface, like those of wading birds and the 

 snow-shoe rabbit. Sand-dwelling lizards and insects similarly are 

 enabled to move over loose sand by toes or legs widened by lateral 

 scales or hairs. Other animals have feet especially adapted for climb- 

 ing trees (squirrels), or for clinging to branches or leaves (tree frogs), 

 or for dealing with other special substrata. 



In contrast to the species requiring rapid locomotion over the land 

 surface is a large group of animals that burrow into the substratum. 

 Many rodents, some birds and reptiles, and a great many insects, as 

 well as other types of invertebrates, are built for effective digging in 

 the ground. Certain species such as the mole, the earthworm, and 

 many insects spend most of their lives underground. All these bur- 

 rowing forms are limited to fegions in which suitable soil conditions 

 exist. For example, a tongue of soil running across the Florida pan- 

 handle that is too dry for the burrowing of crayfish acts as an ecological 

 barrier separating certain west Florida species of crayfish from species 

 limited to areas farther to the east ( Hobbs, 1942 ) , 



The chemical composition of the soil substratum affects animals 

 both directly and indirectly through their food. Land snails with 

 calcareous shells are especially abundant on soils rich in lime, but 

 the abundance of the shell-less slugs is not affected in this way. The 

 shell of one species of Helix was found to weigh 35 per cent of the total 

 weight of the snail in limestone regions but only about 20 per cent of 

 the total weight in areas with soils poor in lime (Hesse, Allee, and 

 Schmidt, 1951, Ch. 20). The bones of mammals likewise are heavier 

 on limestone soils; this is especially true of deer that annually must 

 grow new antlers sometimes weighing as much as 7 kg. It is no acci- 

 dent that strongly built race horses are raised in the bluegrass pas- 



