304 PRINCIPLES OF ANIMAL BIOLOGY 



The colder ocean waters, north and south, have fewer species of 

 animals, but more individuals in a given volume, than do tropical 

 regions. There is a striking similarity of arctic and antarctic animals, 

 the same genera and even species occurring in l)oth oceans. This is 

 presumably accounted for by the fact that there is a connection between 

 them in the cold deep waters of the intervening ocean, which is kept 

 cold by a north-and-south circulation of surface and bottom waters. 

 Another possible explanation is that northern and southern species have 

 evolved independently under the guidance of similar conditions. 



Soil. — Different types of land environment represent different stages 

 in the evolution and concjuest of the earth. Starting with bare rock, 

 the succession is roughly rubble, bare sand, sparse grass and other vege- 

 tation, herbs, shrubs, and trees. The soil may thus be in a variety of 

 conditions, since it develops by weathering and by the action and con- 

 tributions of the vegetation. In texture it may range from very fine 

 particles, as in clay, to coarse stones, as in gravel. In a good loam suit- 

 able for plant growth, about half of the bulk of the soil is made up of 

 spaces between the particles, and these spaces are occupied about equally 

 by air and water. About 10 per cent of the solid matter is derived from 

 plants; the rest is mineral. 



The temperature of the soil varies most at the surface and is nearly 

 constant below a depth of 1 meter. Surface temperature fluctuates 

 much more if the ground is bare than if it is covered by vegetation. In 

 very cold regions the soil may freeze so deep in winter that it is never 

 thawed out in summer; nevertheless, vegetation may gi-ow above this 

 perpetual ice. 



Water may be held loosely in the larger spaces between soil particles, 

 in which case it tends to drain away by its own weight, or it may be 

 retained by capillary action between the fine particles. Even "dry" 

 soil has some moisture adsorbed on the small particles. Silt and clay 

 retain much more moisture than does sand or humus. 



The importance of the soil as an ecological unit may easily be under- 

 estimated, unless it is remembered that most animals spend at least part 

 of their life cycle in the soil. Some animals spend their whole lives 

 there, such as earthworms, some roundworms, and protozoa. Some 

 live in the soil during one stage, such as the grub of May beetles or the 

 pupae of many other insects. Others make their homes in the soil but 

 spend much of their time on or above its surface, as ants and termites. 

 Burrowing in the soil is the common mode of life of moles and shrews, 

 while homes are built in the ground by many other vei'tebrate animals 

 (ground squirrels, ground hogs, mice, etc.). In numbers of individuals, 

 the I'oundworms are the most abundant group, reaching as many as 

 half a billion per acre. 



