126 The Outline of Science 



times of flood. There is no more interesting study in the world 

 than an inquiry into the adaptations by which freshwater animals 

 overcome the difficulties of the situation. We cannot give more 

 than a few illustrations. 



(1) Drought is circumvented by the capacity that many 

 freshwater animals have of lying low and saying nothing. Thus 

 the African mudfish may spend half the year encased in the mud, 

 and many minute crustaceans can survive being dried up for 

 years. (2) Escape from the danger of being frozen hard in the 

 pool is largely due to the almost unique property of water that 

 it expands as it approaches the freezing-point. Thus the colder 

 water rises to the surface and forms or adds to the protecting 

 blanket of ice. The warmer water remains unfrozen at the bot- 

 tom, and the animals live on. (3) The risk of being washed 

 away, e.g. to the sea, is lessened by all sorts of gripping, grap- 

 pling, and anchoring structures, and by shortening the juvenile 

 stages when the risks are greatest. 



V. THE DRY LAND 



Over and over again in the history of animal life there have 

 been attempts to get out of the water on to terra firma, and 

 many of these have been successful, notably those made (1) by 

 worms, (2) by air-breathing Arthropods, and (3) by amphibians. 



In thinking of the conquest of the dry land by animals, we 

 must recognise the indispensable role of plants in preparing the 

 way. The dry ground would have proved too inhospitable had 

 not terrestrial plants begun to establish themselves, affording 

 food, shelter, and humidity. There had to be plants before there 

 could be earthworms, which feed on decaying leaves and the like, 

 but how soon was the debt repaid when the earthworms began 

 their worldwide task of forming vegetable mould, opening up the 

 eartli with their burrows, circulating the soil by means of their 

 fa stints, and bruising the particles in their gizzard certainly the 

 most important mill in the world. 



