420 ANIMAL STUDIES 



as food by sailors, and thus was soon destroyed. It is now 

 known only by its bones preserved in a few museums. The 

 passenger-pigeon of America, which migrated north and 

 south through the Mississippi Valley in flocks of such 

 countless numbers that the sun was darkened as it passed, 

 and which loaded the forest trees of Kentucky with its 

 nests a few decades ago, is now a rare bird, a treasure in 

 the museums. 



335. Fossils. Of all these recently extinct animals, we 

 have preserved to us bones, or stuffed specimens, or eggs, 

 as well as the records of personal observation. But we 

 know of the former existence of thousands and thousands 

 of other animals, now extinct, through remains of another 

 kind. These are either actual remains of bones or other 

 parts preserved intact in soil or rocks, or else, and more 

 commonly, parts of the animals which have been turned 

 into stone, or of which stony casts have been made. All 

 such remains buried by natural causes are called fossils. 

 The process by which they are sometimes changed from 

 animal substance into stone is called petrifaction. 



Fossils may be of three kinds. In the case of recently 

 extinct animals, bones or other parts of the body may be- 

 come buried in the soil and lie there for a long time with- 

 out any change of organic into inorganic matter. Thus 

 fossil insects are found with the bodies preserved intact in 

 amber, a fossil resin from some ancient and extinct pine- 

 tree. Over 800 species of extinct insects are known from 

 amber fossils. The bones of the earliest members of the 

 elephant family, the teeth of extinct sharks, and the shells 

 of extinct mollusks have also been found intact, still com- 

 posed of their original matter. 



In the second kind of fossils the original or organic 

 matter is gone, the organic form and organic structure 

 being preserved in mineral matter. That is, the organic 

 matter has been slowly and exactly replaced by mineral 

 matter. As each particle of organic matter passed away by 



