ITS NATURE AND FUNCTION. 295 



aver ; "but as the forms of vitality "become fewer and lower 

 in kind with, each successive remove in time, and as the 

 Laurentian forms are both very lowly in organisation and 

 scanty in number, inquirers are constrained to believe that 

 they are nearly approaching, if indeed they have not already 

 reached, the first beginnings of life on our planet. Be this 

 as it may, the earliest known forms of vitality occur in the 

 Laurentian system, and it seems something more than a 

 mere coincidence, that these forms should belong at the 

 same time to the very lowest orders that are known to 

 Zoology. Such is all we know of the commencement of 

 life on our globe ; such is the ultimate limit to which geo- 

 logical science has yet been enabled to push her investiga- 

 tions. But while this is incontrovertible as matter of fact, 

 we may believe and all analogy seems to favour the be- 

 lief that life was contemporaneous with the laying down 

 of the first-formed sediments. The external conditions 

 (light, heat, cold, rains, rivers, and seas) that favour the 

 one set of operations are usually those that accompany the 

 other ; and thus, wherever sedimentary strata occur, there 

 also may we expect to find traces of vegetable or animal 

 organisation. If the Laurentian be the earliest formed 

 strata, we have already reached the goal; should others 

 have existed before them, we can merely regard them 

 as the provisional commencement of that long line of 

 vital development which Geology is still labouring to 

 reveal. 



But though ignorant of the origin and commencement of 

 life, we know something of its nature and functions. We 

 perceive that minerals increase by the accretion or external 

 addition of similar matter, but vegetables and animals grow 

 by the internal assimilation of substances which they 

 absorb and convert each into its own proper tissues. Once 

 formed, and the course of the mineral is completed; once 



