384 BIOLOGY 



is only under special conditions that the body of an animal 

 or plant becomes imbedded in the rocks and preserved in the 

 form of a fossil. Incomplete as it is, paleontology has shown 

 us many illuminating facts concerning the earlier life of the 

 world. It has shown that life has been in existence on the 

 earth for many millions of years, although we have no means 

 of determining, even approximately, how many. It has taught 

 that during this long series of ages there has been a constant 

 succession of living things, one type after another making its 

 appearance and giving place to other types. The animals and 

 plants living to-day represent only the last step in this long 

 series, nearly all of the species existing at the present time 

 being of recent origin, some having been in existence only a 

 few thousands, or perhaps even a few hundreds of years, al- 

 though some of our present forms may extend back for hundreds 

 of thousands of years in the past. The immediate predecessors 

 of our present species were organisms much like them, and from 

 them the present forms have doubtless been descended; and 

 preceding these were others, still more remote in time and more 

 unlike the present ones in structure, representing still earlier 

 ancestral forms. 



The general history of any series of types has been approxi- 

 mately as follows: Appearing in a certain part of the world, 

 a group of animals has dispersed itself more or less over the 

 face of the earth, becoming numerous in species and giving 

 rise to a variety of subordinate types. The development has 

 commonly gone on until a climax has been reached, after which 

 the particular type has perhaps remained constant for a time, 

 but eventually declined toward its final extermination. As 

 it disappeared, its place was taken by some other type, better 

 adapted to the new conditions of the changing world. So the 

 progress has gone on age after age, type after type appearing, 

 developing, culminating, and then declining and disappearing. 



One general law in this long progress is manifest. During 

 the whole series of ages there has been a general progress of 



