2S4 THE WORLD OF LIFE 



is sometimes from nine to ten inches. Here, again, we seem 

 to find a tendency to development in size, which has gone on 

 from age to age, till limits have been reached to exceed which 

 threatens the existence of the species. 



The progressive development of many groups of animals 

 affords curious illustrations of this continuous increase in bulk, 

 or in the size of particular organs, till they have actually over- 

 passed the line of permanent safety, and under the first ad- 

 verse conditions have led to extinction. Both 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 ap- 

 parently 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 de- 

 veloped 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 dimensions ever reached by a flying crea- 

 ture, and then the whole group became extinct at a time when 

 a higher type, the birds, w^ere rapidly developing. 



With mammals the case is even more striking, all the ear- 

 liest forms of the Secondary age being quite small; while in 

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

 velop 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 ex- 

 tinct. Others of a lower and more generalised type, but 

 equally bulky, had successively disappeared at the termina- 

 tion 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; tlie latter passing from a horn- 

 less state to one of simple horns, gradually increasing in size 



