DAWN OF LIFE. 179 



development, from the simpler protozoa up through the 

 radiata, articulata, and mollusca. As every class, however, 

 of the invertebrata is represented in Silurian strata, we must, 

 according to this hypothesis, seek for the commencement 

 of the simpler forms, stage by stage, further back among 

 the Cambrian and metamorphic rocks. Such a belief would 

 carry the commencement of life immensely further back in 

 time as far back, indeed, before the Silurian period for the 

 development of the invertebrates, as from the Silurian to the 

 tertiary for the vertebrates ; but as the same ratio cannot 

 possibly be applied to two sub-kingdoms so entirely dis- 

 similar, the idea of a long pre- Silurian cycle of invertebrate 

 life, however plausible, has really little in fact to recom- 

 mend it to our acceptance. We by no means argue for the 

 restriction of life to the Cambrian period, but we must have 

 something more certain than fanciful analogies to carry our 

 convictions any distance beyond these strata. And even 

 the evidence of Fossil life itself is greatly in favour of the 

 belief that at this stage we have reached, or all but reached, 

 the dawn of organised existence. As we ascend in the geo- 

 logical scale, we find life increasing and spreading, stage by 

 stage, into newer and higher forms ; and as we descend we 

 find it decreasing and narrowing to simpler and lowlier as- 

 pects ; and surely we are justified in the inference, that in 

 the few scattered organisms of Cambria, we have all but 

 attained the ultimate limit of vitality. Were matter and 

 life co-dependent we might reasonably argue for their co- 

 existence; but as matter can exist without the manifesta- 

 tion of vitality, and as life appears only in subordination to 

 the material forces, so the one may have existed for ages 

 without necessarily implying the presence of the other. 



And, further, if untold epochs have been spent in the 

 evolution of life from its earliest to its present aspects, it 

 is equally conceivable that cycle after cycle may have rolled 

 by in the elimination of the purely material structure of 



