DIVERSITY OF SILURIAN LIFE. IO5 



time the Silurian age came to an end, nearly all 

 of our existing families of animals were probably 

 present ; but at the beginning they were by no 

 means so abundant. But the fauna of the very 

 earliest primordial rocks was sufficiently diversified 

 to be surprising ; and even with this understanding 

 of the great length of the Silurian age, the difficulty 

 is still very great. 



In explanation, one of two assumptions is neces- 

 sary: either that the time preceding the Silurian 

 age was much greater than the time since then 

 and this is highly improbable ; or that slight modifi- 

 cations at the bottom of a diverging series produce 

 greater results than equal modifications higher up. 

 Starting with a very simple ancestral form at the 

 bottom of the tree, it is evident that slight varia- 

 tions will immediately produce widely different 

 forms, giving rise at once to the great branches 

 of the animal kingdom ; but after somewhat highly 

 specialized forms have arisen, slight variations will 

 have less effect ; just as slight variations in the direc- 

 tion of the growing buds in a very young tree may 

 produce eventually great branches, while similar 

 variations at the ends of these branches, after they 

 are grown, produce only twigs. After groups be- 

 come highly specialized, the tendency is to produce 

 leaves rather than branches in zoological classifica- 

 tion. If this tendency express a fact, we can per- 

 haps get a hint toward the elucidation of the above 

 difficulties. Preceding the Silurian age, enough 

 time must have elapsed since the world was inhab- 

 ited to produce the large branches of the animal 



