202 RELICS OF PRIMEVAL LIFE 



tures we may form on these more problematic points, 

 the observations above detailed appear to establish 

 the following conclusions : — 



First, that in the Laurentian period, as in sub- 

 sequent geological epochs, the Rhizopods 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 de- 

 velopment, in point of magnitude and complexity, 

 unexampled, in so far as yet known, in the succeed- 

 ing ages of the earth's history. This early culmina- 

 tion of the Rhizopods is in accordance with one of 

 the great laws of the succession of living beings, 

 ascertained from the study of the introduction 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- 



