CONSTRUCTION OF THE EARTH. 153 



But for their labors, the tropical forests, bad as they 

 are now with fallen trees, would be a thousand times 

 worse."* 



236. Corals. Coral animals, which are of the class 

 called Polypes, are the most important organic or living 

 agencies that have contributed to the construction of 

 the earth. These animals are very small, most of them 

 exceedingly so, being less than the size of a pin's head. 

 They live in companies together, sometimes each a sep- 

 arate animal, and sometimes united together by a fleshy 

 mass, making what is called a polypidom, or household 

 of polypes. When separate, each sits like a cap on the 

 summit of a column of carbonate of lime, and there takes 

 in its food as it can catch it from the passing water. To 

 this mineral column he is continually adding, and the 

 process is a singular one. The animal is ever dying be- 

 low and growing above ; and as the column may be con- 

 sidered his skeleton, he may be said to be continually 

 leaving dead skeleton below him as he grows. The 

 process is essentially the same, though modified, in the 

 case of the polypidom. In this work each animal does 

 but little, but in the aggregate vast results are accom- 

 plished. In the ages that are past these little animals 

 have made immense additions to the limestone in the 

 earth's crust, and they are continuing their work now. 

 The rocks which result from their work are not really 

 built up by these animals. They furnish the material, 

 which is broken up and changed into limestone, though 

 there are imbedded in many of these rocks the coral 

 forms. And the material is not furnished by these ani- 

 mals alone, for shell-fish and other animals abound wher- 

 eveivthere are corals, and their remains add to the mater 



* I have noticed the earth-worms and ants in this chapter, rather 

 than in the chapter on present changes, partly because they have un- 

 doubtedly been great geological workers in past ages, and partly be- 

 cause it is appropriate to consider their work in connection with the 

 general view of the preparation of the soil for the use of man. 



G2 



