870 LIFE : OUTLINES OF GENERAL BIOLOGY 



ancestry, the more convinced one becomes of the impossibihty of 

 success if plants had not led the way. Plants ensured the food, the 

 moisture, the shelter, without which the dry land would have been 

 altogether too inhospitable for animals. Thus the problem of the 

 origin of land plants has an enhanced interest. 



It seems quite certain that many ages passed before there were 

 any land plants at all. In Cambrian, Ordovician, and Silurian strata 

 there are plenty of traces of seaweeds; but there are no known 

 fossil land plants before the Devonian. Among the earliest are the 

 very interesting Devonian fossils discovered at Rhynie in Aberdeen- 

 shire. Of course it is quite possible that there may have been 

 pioneer land plants long before the Devonian, but of a type too 

 simple to admit of definite fossiUsation. 



The general view in regard to the origin of terrestrial plants is 

 something like this: the simplest plants began in the sea and 

 flourished there for ages ; but some of them, obedient to the universal 

 impulse to press into empty comers, made their pioneering way 

 from shore to estuary, from estuary to river, from river to lake, 

 from lake to swamp and marsh, and thence, at last, began to colonise 

 the dry land. At each station in their ascent some would no doubt 

 settle down and specialise as best they could in relation to the 

 immediate environment, while others would spread onwards, trying, 

 as it were, to find something better. Whether some may not have 

 passed directly from the seashore to the shore-marsh, and thus on 

 to dry land, without serving an apprenticeship in the fresh waters, 

 is a question in detail which may be waived for the present. But 

 the general idea thus sketched is that relatively simple plants, 

 endowed with considerable migrating power, like many of the 

 unicellular algae, did the exploring; and that structural evolution 

 might well advance in the successive stations where they established 

 themselves. One must remember that detached propagative parts of 

 plants would not readily migrate upstream, though spores might be 

 borne by the wind. Fishes may have helped in transport, but there 

 were no plant-distributing birds in those early days. Moreover, there 

 were no true seeds before the Devonian. The general idea seems to 

 be that very simple plants did the travelling; and that, when they 

 reached suitably moist terrestrial resting-places, they proceeded to 

 evolve into organisms like our liverworts, mosses, and ferns, building 

 up structural complexities somewhat similar to those that had 

 already been achieved among seaweeds in salt water, similar yet 

 different, being adapted to the quite novel conditions of terrestrial 

 life. In his Origin of a Land Flora (1908), Bower has sought to show 

 how the exaggeration of the spore-bearing (sporophyte) generation 

 and the repression of the sex-cell-bearing (gametophyte) generation, 

 which is so characteristic of aU flowering plants, would foUow as a^ 

 natural outcome of becoming terrestrial. But the prior question is 



