72 THE SUCCESSION OF ANIMAL FORMS 



All these characters give them an aspect far from embryonic 

 while, as Barrande has pointed out, this advanced position of 

 the group has its significance greatly strengthened by the fact 

 that in early primordial times we have to deal not with one 

 species, but with a vast and highly differentiated group, embrac- 

 ing forms of many and varied subordinate types. As we shall 

 see, these and other early animals may be regarded as of 

 generalized types, but not as embryonic. Here, then, meets us 

 at the outset the fact that in as far as the great groups of annu- 

 lose and molluscous animals are concerned, we can trace these 

 back no farther than to a period in which they appear already 

 highly advanced, much specialized and represented by many 

 diverse forms. Either, therefore, these great groups came in on 

 this high initial plane, or we have scarcely reached half way 

 back in the life-history of our planet. 



We have, here, however, by this one consideration, attained 

 at once to two great and dominant laws regulating the his- 

 tory of life. First, the law of continuity, whereby new forms 

 come in successively, throughout geological time, though, 

 as we shall see, with periods of greater or less frequency. 

 Secondly, the law of specialization of types, whereby general- 

 ized forms are succeeded by those more special, and this pro- 

 bably connected with the growing specialization of the inorganic 

 world. It is this second law which causes the parallelism 

 between the history of successive species and that of the 

 embryo. 



We have already considered the claims which Eozoon and 

 its contemporaries may urge to recognition, as beginnings of 

 life ; but when we ascend from the Laurentian beds, we find 

 ourselves in a barren series of conglomerates, sandstones, and 

 other rocks, indicating shore rather than sea conditions, and 

 remarkably destitute of indications of life. These are the 

 Huronian beds, and possibly other series associated with them. 

 They have afforded spicules of sponges, casts of burrows of 



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