DISCOVERY 



167 



least no direct evidence that Archaean lands were 

 tenanted by plants. Resting upon Archaean rocks, 

 and occupying the next place in the succession of 

 geological formations, are thousands of feet of what are 

 called the Cambrian strata, for the most part con- 

 sisting of material derived from the wear and tear of 

 rocks exposed to the air, which was eventually spread 

 out on the floors of oceans. These Cambrian rocks, 

 with the fossils they contain, teU us much of the 

 animals which lived in the sea, and fiunish some 

 evidence of the occurrence of seaweeds. Of a land 

 vegetation they tell us nothing. The next two periods 

 are known as the Ordovician and Silurian. These 

 two chapters of ancient earth-history are represented 

 by vast thicknesses of marine sedimentary strata, 

 and, particularly the Ordovician period, by a consider- 

 able development of volcanic lavas and ashes of 

 submarine origin. Much of the most picturesque 

 scenery of the Enghsh Lake District is due to mountains 

 composed of volcanic and sedimentary rocks of Ordo- 

 vician age, originally formed under the sea and then 

 upraised as mountain-chains which, through long 

 exposure to denudation, have lost much of their former 

 grandeur. Both the Ordovncian and Silurian rocks, 

 though rich in marine fossils, have hitherto furnished 

 no thoroughly' satisfactory records of land-plants. 



Passing to the Devonian period, the next stage of 

 geological history, we find that in some parts of the 

 world marine strata were built up on the floor of the 

 open sea as in the preceding periods, while in other 

 regions material worn by water and frost from the 

 surface of the land was being piled up on the floors 

 of large freshwater lakes and arms of the sea. Rocks of 

 the latter tj-pe, usually spoken of as Old Red Sandstone, 

 are a characteristic feature of much of the country 

 encirchng the Highlands of Scotland ; they occur 

 also in the Shetland Isles, Scandinavia, Ireland, and 

 other parts of the world. It is in Devonian or Old Red 

 Sandstone strata, deposited in lakes or enclosed seas, 

 that representatives of the oldest known land-vegeta- 

 tion hav^e been discovered. 



By far the most important samples of this ancient 

 vegetation of the land are the petrified plants, beauti- 

 fully preserved, with their most delicate cells intact, 

 which were discovered a few years ago by Dr. Mackie, 

 of Elgin, in certain siliceous Devonian beds in Aber- 

 deenshire. Detailed descriptions of some of these 

 fossils have been published in the Transactions of the 

 Royal Society of Edinburgh, by Dr. Kidston and 

 Professor Lang (1917, 1920). Many other Devonian 

 plants have been described from different parts of 

 Scotland, from Ireland, Devonshire, Canada, the 

 United States, Norway, Bohemia, Germany, and 

 elsewhere. 



It is by no means improbable that plants existed 



on the land during at least part of the inconceivably 

 long period of earth-history which had passed before 

 the Devonian period began. All that can be said is 

 that the records of pre-Devonian rocks tell us practically 

 nothing of any land-vegetation there may have been. 

 The question now before us is, where were the first 

 land-plants evolved ? Were they evolved from simpler 

 forms which originated on an exposed land-surface, or 

 were the earUest land-plants descendants of Alga 

 living in the sea ? Some writers favour the view that 

 plant-life began in freshwater pools on a primeval 

 continent, and it is often assumed that some existing 

 freshwater members of the Alga;, if not themselves the 

 ancestors of the more complex inhabitants of the dry 

 land, at least indicate the lines followed by evolution. 

 Another view is that plant-life began in the sea ; Algae 

 inhabiting the shallow coastal regions migrated, it is 

 thought, through rivers to inland waters and eventu- 

 ally established themselves on the land, thus forming 

 the starting-point of the complex terrestrial plants. 

 Dr. Church suggests a different course of evolution : 

 " The beginnings of Botany are in the sea " ; he believes 

 that before continents and oceans divided the world 

 between them, the earth's surface was covered by 

 water ; a world-ocean two miles deep wrapped com- 

 pletely round the solid crust. This is, of course, an 

 assumption ; and though it is directly opposed to 

 the opinions usually expressed by geologists, the 

 possibility of a complete covering of water is not, so 

 I am informed, beyond the range of possibility from 

 an astronomical point of view. In this original ocean 

 life was evolved — the method of evolution does not 

 now concern us ; during a long period the earliest 

 organisms increased in complexity, and at length 

 single-ceUed plants were produced capable of manu- 

 facturing their own food, like the small aquatic green 

 plants of to-day, that build up their substance from 

 salts and carbon dioxide gas absorbed from a watery 

 medium. Thus, the world-ocean, not as yet penetrated 

 by any part of the earth's crust, became tenanted by 

 hosts of unicellular and free-swimming plants. At a 

 later stage, in response to those forces which have 

 been constantly acting on the mass of the earth, 

 portions of the crust rose to within 150 to 100 fathoms 

 of the surface of the ocean and came under the in- 

 fluence of the sun's rays. On this feebly illuminated 

 floor of the shallow sea some of the free-floating plants 

 adopted a sedentary habit, and by slow degrees from 

 unicellular forms were evolved multicellular and more 

 complex seaweeds, which were anchored to the rocks 

 by root-like holdfasts. As time elapsed, these anchored 

 forms developed into larger seaweeds, surpassing, it 

 is believed, both in their structural complexity and in 

 their reproductive mechanisms, even the highest types 

 of existing brown, red, or green marine Algae. The 



