264 THE WORLD OF LIFE CHAP. 



reptiles and mammals originated in creatures of small 

 size which gradually increased in bulk, in certain types, 

 till they suddenly became exterminated. In the former 

 class the increase was apparently rapid, till the hugest land- 

 animals that ever lived appeared upon the earth the 

 Dinosauria of the Jurassic and Cretaceous periods, already 

 described. Many of them also developed strange horns and 

 teeth ; and these, too, when they reached their maximum, also 

 suddenly disappeared. Flying reptiles the Pterodactyles 

 also began as small animals and continually increased, till 

 those of the period of our Chalk attained the greatest dimen- 

 sions ever reached by a flying creature, and then the whole 

 group became extinct at a time when a higher type, the 

 birds, were rapidly developing. 



With mammals the case is even more striking, all the 

 earliest forms of the Secondary age being quite small ; while 

 in the Tertiary period they began to increase in size and to 

 develop into a great variety of types of structure ; till, in an 

 age just previous to our own, such exceedingly diverse 

 groups as the marsupials, the sloths, the elephants, the 

 camels, and the deer, all reached their maximum of size and 

 variety of strange forms, the most developed of which then 

 became extinct. Others of a lower and more generalised 

 type, but equally bulky, had successively disappeared at the 

 termination of each subdivision of the Tertiary age. It is 

 here that we can trace the specialisation and increase in size 

 of the horse-tribe and of the deer ; the latter passing from a 

 hornless state to one of simple horns, gradually increasing in 

 size and complexity of branching, till they culminated in the 

 great Irish elk, which was the contemporary of the mammoth 

 and man in our own country. 



Dr. A. S. Woodward, keeper of Geology in the British 

 Museum, discussed this curious phenomenon in his presi- 

 dential address to the Geological Section of the British 

 Association in 1909 ; and a few extracts will show how 

 widespread are these facts, and the great interest they have 

 excited. After sketching out the whole course of animal 

 development, and showing how universal is the law (much 

 emphasised by Darwin), that the higher form of one group 

 never developed from similar forms of a preceding lower 



