1 60 E VOL UTION OF TO-DA Y. 



longer a surprise that when the Silurian age opened 

 they were all found well developed. It is more of 

 a surprise that we do not find the vertebrates as 

 well developed as the rest. 



It is evident that we cannot trace the common 

 embryology of animals beyond this gastrula. If we 

 wish to go farther we must follow the sub-kingdoms 

 separately. The difficulty increases as we attempt 

 to take another step. In some way this gastrula 

 was modified in various directions to give rise to 

 the various sub-kingdoms, but the embryological 

 record has been so much modified as to make it at 

 present hardly possible to follow the history. In a 

 few cases it can be done (coelenterates, annelids, 

 mollusks) but as a rule it is impossible ; in most 

 cases we find that our evidence leaves a chasm 

 between the gastrula and the earliest form which 

 can be regarded as showing an approximation 

 toward the anatomy of the sub-kingdom to which 

 the embryo belongs. All sorts of suggestions have 

 been offered as to the filling of this chasm by the- 

 ories regarding the exact manner in which the gas- 

 trula was elongated or expanded to form the various 

 types arising. The vertebrates have received the 

 most attention, and here theories are particularly 

 abundant. But every thing is as yet only hypotheti- 

 cal and we need not take the trouble to examine the 

 question. 



After passing this chasm we can once more reach 

 sure footing. We now find that embryos have as- 

 sumed various forms, each conforming to the type 

 of its own sub-kingdom. It is not our purpose to 



