214 TOWARDS THE HUMAN FORM 



in the Silurian presupposes the existence at some earlier epoch, 

 either the Tower Silurian or the Cambrian, of Crustaceans 

 belonging to these two orders. They have not been found. 

 Yet such gaps occur in all groups ; indeed, a group sometimes 

 appears to nourish in one epoch and then to vanish, only to 

 reappear later on in very much the same forms, and occasionally 

 to persist even till the present day. It is evident that these 

 disappearances and reappearances are only apparent. When 

 they are not merely the result of our own imperfect investiga- 

 tions they simply hide from us migrations that have taken 

 place owing to some alteration either in the composition of 

 the water, its depth, the nature of the deposits at the bottom, 

 or the direction of the currents, and so forth. Such phenomena 

 are neither uncommon nor in any way mysterious. Only a few 

 years ago a series of hard winters in the bay of Saint-Vaast- 

 la-Hougue, rendered famous by the researches of Henri 

 Milne-Edwards, de Quatrefages, Claparede, Grube, and many 

 others, caused the disappearance of the Comatulids and Asterina, 

 which lived there in great numbers, and brought in exchange 

 various northern species, hitherto unknown in these latitudes. 

 The old fauna has not even yet been restored. This represents 

 on a small scale what often happened during the great geological 

 periods. 



Why did the Trilobites disappear ? We saw that the 

 existence of Myriapods and Insects at the close of the Primary 

 implied the presence in the seas of that epoch of higher 

 Crustaceans whose fossil remains do not become numerous 

 till we get into the Secondary. With their very complete 

 buccal armature, their stout appendages, adapted in some cases 

 to walking and in others to swimming, these creatures should 

 have been able to supplant the Trilobites easily, either by 

 preying upon them or simply by competing with them for 

 food. In Victor Hugo's words, " Ceci devait tuer cela." x 

 Thus the mere demonstration of the existence of a group of 

 animals at some definite epoch can be extraordinarily 

 suggestive. 



Significant facts of this kind are not lacking. Two genera 

 of Scorpions have been found in the Silurian (p. 169). This 

 is not at all surprising, since Scorpions are very closely related 



1 The second had to kill the first. 



