398 ELEMENTS OF BIOLOGY 



the geologists have divided the lapse of time since the earth as- 

 sumed something like its present form into five eras (Fig, 221). 

 There is some disagreement as to the period of time covered by 

 these eras but that does not concern us here. It is sufficient to say 

 that millions of years have passed since life first appeared on the 

 earth. The accompanying table (Fig. 221) summarizes this dis- 

 tribution of animal life in time. It will be noted that the remains of 

 Man are confined to the most recent era. The period marked by 

 the presence of Man on the earth is therefore very short as com- 

 pared with the time preceding his appearance. If we in turn com- 

 pare the period of recorded history of Man, that is, the length of 

 time since the human intelligence has been sufficiently developed 

 to make observations and keep records, with the whole period of 

 Man and again with the vast time that has preceded his appear- 

 ance, it becomes clear to us why so little of the process of evolution 

 has come under direct observation, why so few changes have been 

 observed, and why our conclusions concerning the origin of any 

 given form must be drawn by inference from fossil remains and by 

 comparison with other forms. The most fruitful field of investi- 

 gation of many of the concepts of Biology is by experiments which 

 duplicate the processes of life under controlled conditions. But it is 

 obvious that the effect of the long periods of the earth's history on 

 living things cannot be repeated by Man under controlled condi- 

 tions. It is the task of the paleontologist to read the record of 

 Nature's great experiment; all these records point to a conclusion 

 that is inescapable to a rational mind, namely, that the animals of 

 the past are the ancestors of the animals of the present, and that 

 the animals of the present have been evolved by descent with 

 change. 



The records of the life of bygone periods are incomplete; prob- 

 ably very many varieties of plants and animals have existed of which 

 no traces have been found. Soft-bodied animals as they decay 



