THE SUCCESSION OF ANIMAL FORMS 175 



and Univalve Mollusks and Crustaceans, or in other words, all 

 the leading groups of invertebrate animals that we find in the 

 sea at present. Of these the dominant group is the Crustaceans, 

 including Trilobites, numbering one-third of the whole ; and 

 these with the univalve Mollusks and the Brachiopods constitute 

 the majority, the other groups having comparatively few species. 

 What a marvellous incoming of life is here ! Walcott may 

 well say that on the theory of gradual development we must 

 suppose that life existed at a period far before the Cambrian 

 as far, indeed, as the Cambrian is before our own time. But 

 this would mean that we know only half of the history of life ; 

 and perhaps it is more reasonable to suppose that when the 

 conditions became favourable, it came in with a rush. 



Before considering the other laws that may be inferred from 

 these facts, however, let us in imagination transfer ourselves 

 back to the Primordial age, and suppose that we have in our 

 hands a living specimen of one of the larger Trilobites, recently 

 taken from the sea, flapping vigorously its great tail, and full of 

 life and energy ; an animal larger and heavier than the modern 

 king-crab of our shores, furnished with all the complexity of 

 external parts for which the crustaceans are so remarkable, and 

 no doubt with instincts and feelings and modes of action as pro- 

 nounced as those of its modern allies, and, if Woodward's views 

 are correct, on a higher plane of rank than the king-crab itself, 

 inasmuch as it is a composite type connecting Limuli with 

 Isopods, and even with scorpions. We have obviously here, 

 in the appearance of this great Crustacean or Arachnoid, a repe- 

 tition of the facts which we met with in Eozoon ; but how vast 

 the interval between them in geological time, and in zoological 

 rank ! Standing in the presence of this testimony, I think it 

 is only right to say that we possess no causal solution of the 

 appearance of these early forms of life ; but in tracing them 

 and their successors upward through the succeeding ages, we 

 may hope at least to reach some expressions of the laws of 

 9* 



