K.— BOTANY. 201 



imbued with a determination to co-operate in taking advantage of the 

 opportunities, which are theirs, of contributing to a fuller knowledge of 

 the evolution of the plant-world than it is possible to obtain from the 

 records of the rocks on the other side of the equator. 



It may be helpful as a preliminary to my treatment of certain aspects 

 of plant-life in former ages to glance at a table of contents of the geological 

 history of the world : we can better appreciate scenes from the past if we 

 think of them in relation to their respective places in the sequence of events 

 registered in the earth's crust. 



The Eaklier Chapters of the History of the Plant World. 



It is important to remember, especially important when we are trying 

 to follow the course of evolution in the organic world, that the rocks which 

 have furnished the earliest known remains of plants are separated from 

 the oldest known part of the earth's crust by thousands of feet of strata 

 and by some hundreds of millions of years. The foundation stones of the 

 world in the strict sense are unknown : we are still unable to answer the 

 question — ' Whereupon are the foundations thereof laid ? 



The crystalline rocks classed by geologists as Archtean represent in- 

 conceivably ancient land-surfaces on which were accumulated vast piles 

 of detrital material furnished by agents of erosion, and from time to time 

 products of volcanic activity. Plants may have lived on the Archaean, or 

 Pre-Cambrian, continents ; they probably did, but as yet we have no 

 certain knowledge of them. We may think of an azoic world, or of a 

 primeval ocean—' the image of eternity ' — pregnant with the first germs 

 of plant-life which in later ages developed into the ancestors of terrestrial 

 vegetation, or our imagination may enable us to picture a Pre-Cambrian 

 land occupied by colonies of primitive plants simpler than any so far 

 discovered in the older Palaeozoic strata. Passing higher in the geological 

 series to the marine sediments and associated lavas and volcanic ash in- 

 cluded in the Cambrian, Ordovician, and Silurian systems, we find clear 

 evidence of the existence of lime-secreting Algse, the precursors of some 

 of the modern reef-forming seaweeds, and, in Silurian strata, a few traces 

 of plants which probably lived on dry land. It is true to say that as yet 

 we know practically nothing of the terrestrial vegetation of the world before 

 the beginning of the Devonian period. The lapse of time represented by 

 that portion of the earth's crust comprised within the Pre-Cambrian, 

 Cambrian, Ordovician, and Silurian periods is much longer than the 

 duration of all the other geological periods put together. What is the 

 story of evolution hidden in the Pre-Cambrian and in the earlier 

 Palaeozoic formations ? This is a question which appeals with especial 

 force to the imagination : though it is too much to expect that we shall 

 «ver discover the earliest links in the chain of life, we may with confidence 

 expect to find remains of pre-Devonian terrestrial plants which, I venture 

 to think, will surprise us by their relatively high level of organisation. 

 The more we know of the older floras, the more difficult it becomes to 

 form a clear conception of the course of evolution of the plant-world. 

 We are prejudiced in favour of generalised types and primitive ancestral 

 forms, but while among the earliest known members of the plant-kingdom 



