134 THE DAWN OP LIFE. 



" First, that in the Laurentian period, as in subse- 

 quent geological epochs, the Ehizopods were important 

 agents in the accumulation of beds of limestone ; and 

 secondly, that in this early period these low forms of 

 animal life attained to a development, in point of mag- 

 nitude and complexity, unexampled, in so far as yet 

 known, in the succeeding ages of the earth's history. 

 This early culmination of the Rhizopods is in accord- 

 ance with one of the great laws of the succession of 

 living beings, ascertained from the study of the intro- 

 duction and progress of other groups ; and, should it 

 prove that these great Protozoans were really the 

 dominant type of animals in the Laurentian period, 

 this fact might be regarded as an indication that in 

 these ancient rocks we may actually have the records 

 of the first appearance of animal life on our planet." 



With reference to the first of the above heads, I 

 have now to state that it seems quite certain that the 

 upper and younger portions of the masses of Eozoon 

 often passed into the acervuline form, and the period 

 in which this change took place seems to have de- 

 pended on circumstances. In some specimens there 

 are only a few regular layers, and then a heap of ir- 

 regular cells. In other cases a hundred or more 

 regular layers were formed; but even in this case 

 little groups of irregular cells occurred at certain 

 points near the surface. This may be seen in plate 

 III. I have also found some masses clearly not frag- 

 mental which consist altogether of acervuline cells. A 

 specimen of this kind is represented in fig. 31. It is 



