8 



THE HOESE 



pigs, the outer toes of the horse's foot, the entire hind 

 limbs of porpoises, &c. (3) The modification of the 

 form, size, and relation of parts — as the immense 

 development of the tusks in the walrus and male musk- 

 deer, the complicated foldings of the grinding teeth of 

 elephants, &c. 



In tracing out any series of gradual modifications 

 following each other in a regular chronological sequence, 

 as w r e are sometimes fortunate enough to be able to do, 1 

 we find that progress is usually from the general to the 

 special. It must not, however, be supposed from this 

 statement that all animals living in ancient times were 

 more generalised in character than many now existing. 

 On the contrary, many of the extinct forms, even those 

 of quite early periods, were in some portion of their 

 structure very highly specialised. In fact, high speciali- 

 sation almost invariably leads ultimately to extinction, 

 because it results from adaptation to particular con- 

 ditions, which may become changed in course of time, 

 and then the animals which have become adapted 

 exclusively for life under those conditions perish, while 

 those animals that retain more general characters readily 

 adapt themselves to the altered circumstances. The 

 commonplace, average sort of creatures are thus often the 



1 Many such instances are described in an interesting series of 

 works, entitled Les Eneliainemcnts du Monde Animal dans les Temps 

 Geologiques, by Professor Albert Gaudry, Paris, 1878-90. 



