SOUTH DAKOTA SCHOOL OF MINES 65 



particle by new mineral matter of some kind, particularly 

 silica or pyrite, then they become petrifactions. Sometimes 

 only the form, or the impression of the original parts are 

 preserved, hence the terms molds and casts. Occasionally 

 the relics are limited to footprints, or trails, or burrows, or 

 borings or eggs. 



Animals living in the water or frequenting marshy 

 places for food and drink are more easily and more quickly 

 buried beneath sediments, hence their fossils are usually 

 more abundant. The bodies of dry land animals are subjected 

 to the vicissitudes of sun and rain and wind, and frost, and 

 are often feasted upon by scavenger birds and beasts and 

 insects. Furthermore their burial is commonly brought 

 about only during flood season. All of these tend to the 

 destruction or dismemberment of the various parts. Again, 

 even if once nicely buried, they may later be obliterated by 

 metamorphism or be destroyed by disintegrating and de- 

 nuding agencies. As a result of all this, the history of cer- 

 tain groups of animals is meagre in the extreme and doubt- 

 less hordes of species have left no worthy evidence of their 

 ever having lived. 



EXTINCTION, EVOLUTION AND DISTRIBUTION OF 



ANIMALS 



The progress of animal organisms is constantly directed 

 toward the goal of perfection. Each individual shares in 

 the improvement but the perfection to be attained consists 

 not so much in the exquisite relation the various organs 

 bear to one another as it does in the harmony that the ani- 

 mal in all its characters shows to its environment. 



When life began, and how, no one knows. It is evident 

 that in the beginning it was represented by very simple 

 forms. These, because of varying conditions, were followed 

 in orderly sequence by creatures of growing complexity. 

 All animals pass through innumerable vicissitudes and 

 existence is a constant struggle. Those best fitted to meet 

 difficulties tend to survive and leave posterity. It thus 

 happens that advantageous variations are perpetuated and 

 those of less use are eliminated. In this way changes oc- 

 cur, characters are modified, and life forms sooner or later 

 take on an appearance and a nature quite different from 

 their ancestors. 



