94 THE LIVING WORLD. 



Upon looking at the world during the Silurian (2) 

 age, we are immediately struck with the surprisingly 

 great diversity of its animal life. The divergence of 

 types of animals, for which we have found evidence 

 in the embryology, had already taken place. The 

 Gastraea had given rise to a number of lines of 

 descendants, all of which had more or less com- 

 pletely lost the original Gastraea characters, and had 

 taken the characters of the animal types of to-day. 

 Not only had the large types made their appearance, 

 but there had already been time enough for most of 

 them to diverge still further, and produce their char- 

 acteristic classes. 



The diversity of life found in the Silurian (2) age 

 has greatly surprised geologists. This surprise is, 

 however, disappearing, as we learn from embryology 

 of the rapid divergence of types at the beginning of 

 life, and as we remember the long blank Archean (i) 

 age, during which life existed without leaving any 

 distinct traces of itself. It must be remembered 

 also that the Silurian (2) age itself was of very long 

 duration, and many of the forms of animal life 

 attributed to this period were not in existence at the 

 beginning of the era, but were developed during its 

 progress. The scantiness of the remains makes it, 

 however, impossible to determine with any degree 

 of probability which of the animal forms came in 

 later. In our study of this early period of life's his- 

 tory, we will consider the age a unit, always under- 

 standing that it is a unit of immeasurable duration, 

 and if we had been able to watch its progress, many 

 of the forms which appear so suddenly would have 



