86 The Outline of Science 



worm is a burrower which eats its way through the soil, the centi- 

 pede's long body is supported by numerous hard legs, and the 

 snake pushes itself along by means of the large ventral scales to 

 which the lower ends of very numerous ribs are attached. 



Methods of Mastering the Difficulties of Terrestrial Life 



A great restriction attendant on the invasion of the dry land 

 is that locomotion becomes limited to one plane, namely, the 

 surface of the earth. This is in great contrast to what is true in 

 the water, where the animal can move up or down, to right or to 

 left, at any angle and in three dimensions. It surely follows from 

 this that the movements of land animals must be rapid and pre- 

 cise, unless, indeed, safety is secured in some other way. Hence 

 it is easy to understand why most land animals have very finely 

 developed striped muscles, and why a beetle running on the 

 ground has far more numerous muscles than a lobster swimming 

 in the sea. 



Land animals were also handicapped by the risks of drought 

 and of frost, but these were met by defences of the most diverse 

 description, from the hairs of woolly caterpillars to the fur of 

 mammals, from the carapace of tortoises to the armour of arma- 

 dillos. In other cases, it is hardly necessary to say, the difficulties 

 may be met in other ways, as frogs meet the winter by falling into 

 a lethargic state in some secluded retreat. 



Another consequence of getting on to dry land is that the 

 e gg s or young can no longer be set free anyhow, as is possible 

 when the animal is surrounded by water, which is in itself more 

 or less of a cradle. If the eggs were laid or the young liberated 

 on dry ground, the chances are many that they would be 

 dried up or devoured. So there are numerous ways in which land 

 animals secure the safety of their young, e.g. by burying them in 

 the ground, or by hiding them in nests, or by carrying them about 

 for a prolonged period either before or after birth. This may 

 mean great safety for the young, this may make it possible to have 





