122 THE DAWN OF LIFE 



sessile on the bottom, grew by the addition of successive 

 chambers, in the same manner with the smaller species.^ 



Let us, then, examine the structure of Eozoon, taking a 

 typical specimen, as we find it in the limestone of Grenville or 

 Petite Nation. In such specimens the skeleton of the animal, 

 is represented by a white crystalline marble, the cavities of the 

 cells by green serpentine, the mode of whose introduction we 

 shall have to consider in the sequel. The lowest layer of ser- 

 pentine represents the first gelatinous coat of animal matter 

 which grew upon the bottom, and which, if we could have 

 seen it before any shell was formed upon its surface, must have 

 resembled a minute patch of living slime. On this primary 

 layer grew a delicate calcareous shell, perforated by innumer- 

 able minute tubuli, and resting on the slimy matter of the 

 animal, though supported also by some perpendicular plates or 

 septa. Upon this again was built up, in order to strengthen it, 

 a thickening or supplemental skeleton, more dense, and desti- 

 tute of fine tubuli, but traversed by branching canals, through 

 which the soft gelatinous matter could pass' for the nourish- 

 ment of the skeleton itself, and the extension of pseudopods be- 

 yond it. (Figs. 11,12.) So was formed the first layer of Eozoon, 

 which probably was at its beginning only of very small dimen- 

 sions. On this the process of growth of successive layers of 

 animal sarcode and of calcareous skeleton was repeated again 

 and again, till in some cases even a hundred or more layers 

 were formed (nature-print. Chap. VL) As the process went on, 

 however, the vitality of the organism became exhausted, prob- 

 ably by the deficient nourishment of the central and lower 

 layers making greater and greater demands on those above, 

 and so the succeeding layers became thinner, and less sup- 

 plemental skeleton was developed. Finally, toward the top, 

 the regular arrangement in layers was abandoned, and the cells 



^ I refer to some of the Stromatoporse of the Silurian and the Cryptozoon 

 of the Cambrian. See note appended to this chapter. 



