HISTORY OF THE EARTH 291 



kind seems to be permanent. Yet we know that by skillful 

 manipulation a new variety of fruit tree or a new breed of 

 poultry may be produced from the original familiar forms. 

 Such changes have been going on under natural conditions 

 through all the ages since life began, though more slowly; 

 and these changes, like the slow wear of the rivers and waves, 

 have brought about great transformations. 



If we examine the most recently formed strata of mud and 

 sand, we may find in them the shells of oysters, corals, and 

 other water-inhabiting animals hardly to be distinguished 

 from those which live in the ocean today. But in much older 

 rocks the shells we might discover would bear only a faint 

 resemblance to the existing species. All of the species of 

 animals which lived when those sediments were being de- 

 posited may have long since become extinct and have been 

 replaced by newer types more like those of the present. 

 The shells and other traces of animals and plants preserved 

 in the rocks are called fossils; and, as we shall see, fossils are 

 of great importance because they enable us to trace the prog- 

 ress of life from one stage of its existence to another. When 

 geologists first discovered that in a given series of strata the 

 fossils in the lower layers were distinctly different from those 

 in the upper, they supposed that the earlier animals had been 

 destroyed by some great catastrophe, and that a new set had 

 then been created to replace them. Soon, however, it became 

 clear that the animals in the upper beds were distinctly related 

 to those in the lower, and that the differences between them 

 are chiefly matters of degree. The belief thus became estab- 

 lished that the modern animals and plants have descended 

 from older and older species by a series of very slow changes 

 occupying millions of years. 



Each individual animal and plant to-day begins its existence 

 as a single minute cell, which, as it grows, divides and sub- 

 divides many times until it produces the many cells which 

 the adult body contains. There is every reason io believe 

 that, like the individual animal, the whole animal kingdom has 



