828 LIFE : OUTLINES OF GENERAL BIOLOGY 



Ordovician and Silurian seas. They probably fed in great part on 

 Trilobites. 



Another feature of the Ordovician Period was the first appearance 

 of plants higher than seaweeds, some of them perhaps from marsh- 

 land, with hints of wood- vessels. 



Silurian. — The later part of the Silurian Period was marked by 

 uplifting crust-movements and by aridity, which was continued into 

 the Devonian. With the cumulative drought may be associated an 

 early colonisation of the dry land, and we think of the Silurian as 

 the time of the beginning of a terrestrial fauna. Thus there is a 

 primitive fossil Millipede, Archidesmus, from the Scottish Lower 

 Silurian, which perhaps deserves to be called the oldest known 

 thoroughly terrestrial animal. It does not show the doubling of the 

 body-rings that marks ordinary Millipedes. Two primitive scorpions 

 are also known from the Silurian, but their surroundings, as well as 

 some structural details, suggest that these had already become 

 secondarily aquatic. They had probably relapsed from terrestrial 

 ancestors of yet earher date. Also noteworthy in the Silurian seas 

 were the pioneers of the King-crab race, now represented by the 

 single genus Limulus. The seas were richly peopled, and there was 

 an especial abundance of corals. In fresh waters there was probably 

 an emergence of pioneer Lung-fishes. 



Devonian. — ^The onset of aridity in the late Silurian abated 

 through a great part of the Devonian Period, yet seems to have been 

 characteristic throughout, and even to have risen again to great 

 severity towards its close. Aridity was the climatic keynote; hence, 

 while we mostly think of the Devonian as above all an age of fishes, 

 a yet stronger interest to the evolutionist is in the way the aridity 

 seems not only to have limited animal life on land, but also largely 

 to have stimulated it. For it is from this period that we have to 

 date the peculiarly important advance beyond ordinarily gilled 

 fishes to also lunged ones, notably the Dipnoids, and even the 

 emergence of Amphibians. So our familiar experience of gilled tad- 

 pole changing to well lunged and even vociferous frog is a veritable 

 condensation of the main advance of these Devonian times. 



The dry Devonian climate reduced the extent of rivers and lakes, 

 and the frequent drying-up of pools must have prompted, indeed 

 often compelled, the lunged fishes to make the best of the drying 

 land, and to use their simply shaped fore and aft paired fins for 

 support and locomotion. Moreover, the aeration of fresh waters must 

 have been lessened by lack of streams, and this would favour lung 

 respiration by gulping air at the surface, as Polypterus has still to 

 do. Thus it is not surprising that the first recorded lung-fish appears 

 in the Lower Devonian, and that the first digitate footprint, made 

 by the pioneer Amphibian, Thinopus antiquus, should date from 

 the Upper Devonian. The original of this most eloquent of tracks 



