182 GENERAL ZOOLOGY 



reptiles, and birds are found, indicating that these forms 

 came into existence later than the fishes. Finally, mammals, 

 the highest creatures of the animal kingdom, made their ap- 

 pearance, as shown by the fact that their fossil remains are 

 found in late deposits but not in early ones. It has been 

 well stated that the history of life on the earth has been 

 written in tablets of stone. These give us an accurate, 

 though incomplete, record of the fact that through the ages 

 life upon the earth has been incessantly changing. Species 

 of one age are different from, yet show undeniable relation- 

 ship to, those of the age before. Age upon age has witnessed 

 a gradual increase in the complexity of the animal popula- 

 tion. The oldest fossil rocks contain nothing but the simplest 

 types of animals. The most recent contain all the highest 

 in addition to the lower forms. At intervals between these 

 extremes we can point with certainty to the age when each 

 group of animals first made its appearance. Evidently 

 throughout the past history of the earth some factors have 

 been at work, causing one species, or group, to give rise to 

 different forms. There has been progress in this procession 

 from the simpler to the more complex types of body. This 

 progress is what is ordinarily called evolution. Evolution is 

 a principle that seems to operate throughout the entire uni- 

 verse. All that the idea of evolution necessarily means is 

 this progressive change, the evidences of which are all about 

 us. When we go beyond this observable fact of a changing 

 universe into the question of what has brought about the 

 changes, we have entered another field. All scientists at the 

 present time agree that evolution is a fact. Few agree in 

 their attempts to explain how evolution operates. 



Natural Selection. Charles Darwin worked out a theory 

 to explain how evolution operates. This theory has been 

 so much discussed and criticized that the average person 

 thinks that Darwin's theory of natural selection embodies 



