48 The Make-Up and the Wor\ings of the Earth 



(2) Many forms of plants and animals appeared at suc- 

 cessive periods, multiplied and spread over great areas, and 

 finally vanished completely; and other forms have persisted 

 almost unchanged from the time of their first appearance. 



(3) The forms that appeared in earlier times were of 

 simpler types than the living plants and animals of later 

 periods; and with the passing of time the successive plant and 

 animal inhabitants of the earth approached more and more 

 to those forms living at the present time. 



(4) The succession of life forms and their distribution 

 are such as to agree with a theory of descent with modifica- 

 tion; but while the evidence shows that there has been suc- 

 cession with modification, from the very nature of the 

 material, it cannot prove that there has been descent as well 

 as succession. 



So confident have students of fossils become of the gen- 

 eral facts observed and of the theory of descent-with-modi- 

 fications, that they have repeatedly predicted the finding of 

 fossils with particular characteristics which had not yet been 

 seen by anyone. From a study of Eohippus and of recent 

 fossils representing the horse family, for example, it was possi- 

 ble to predict that there would be found fossils having inter- 

 mediate characteristics; and many such were actually dis- 

 covered in later explorations. This might be compared to 

 the effectiveness of theoretical deductions illustrated by the 

 discovery of the planet Neptune in 1846, the existence of 

 this previously unknown planet having been deduced from 

 the irregularities in the movements of Uranus. Any theory 

 that enables us to predict in advance of direct knowledge, 

 especially in matters that do not themselves yield advance 

 information, deserves serious consideration of its claim to be 

 in accord with reality. 



