216 THE WORLD OF LIFE 



are in many respects strikingly parallel. We find in the 

 Carboniferous series of rocks a storing-up of vast masses of 

 vegetable matter in the form of coal, which is unique in the 

 whole past history of the earth, and this was at a time when 

 the only land vertebrates were archaic forms of amphibians. 

 Almost immediately after the deposit was completed true 

 reptiles appeared all over the earth, and rapidly developed 

 into that " Age of Reptiles " which is perhaps the greatest 

 marvel of geological history. Birds and Mammalia also 

 started into life, apparently branching off from some common 

 stock with the reptiles. Then, during that blank in the 

 record separating the Secondary from the Tertiary era, the 

 whole of this vast teeming mass of reptilian life totally dis- 

 appeared, with the two exceptions of the crocodiles and 

 the tortoises, which have continued to maintain themselves 

 till our own day, while true lizards and snakes, which are 

 not known in earlier times, became the predominant forms 

 of reptilian life. It was during the same blank period of 

 the geological record that mammals and birds sprang into 

 vigorous and diversified life, just as the reptiles had done 

 during the blank between the Primary and Secondary eras. 

 To complete the great series of life-changes (perhaps as a 

 necessary preparation for them), plants underwent a similar 

 transformation ; the prominent Cryptogams, Conifers, and 

 Cycads of the Secondary era gave way towards its close to 

 higher flowering plants, which thenceforth took the first 

 place, and now form probably fully 99 per cent of the whole 

 mass of vegetation, with a variety of nourishing products, in 

 foliage, fruit, and flower, never before available. 



Now here we have a tremendous series of special 

 developments of life-forms simultaneous in all parts of the 

 earth, affecting both plants and animals, insects and verte- 

 brates, whether living on land, in the water, or in the air, all 

 contemporaneous in a general sense, and all determining the 

 transition from a lower to a very much higher grade of 

 organisation. Just as in the first such great step in advance 

 from the " age of fishes " to the " age of reptiles " we see 

 reason to connect it with the change from a more carbonised 

 to a more oxygenated atmosphere, produced by the locking 

 up of so much carbon in the great coal-fields of the world ; 



