NATURAL SELECTION in 



There is also to be seen in the museum the restoration of a 

 tortoise, fossil remains of which were found in the Siwalik Hills 

 in India. This representative of the Pliocene period was, with- 

 out his head and neck, about 8 feet in length ; the Aldabra 

 tortoise, with head and neck, measures about 5 feet in length. 

 Probably this ancient form required even more years to grow to 

 his full size, and enjoyed even a longer span of life than the 

 comparatively small modern forms. But, however generous our 

 estimate, how paltry and insignificant is his term of years 

 compared with that of the giant Sequoia ! For an animal such 

 longevity is impossible. He cannot, like a tree, fix himself in 

 one place and there find food and all that he requires. He must, 

 unless he is a mere parasite, be constantly on the move. He 

 must not only avoid being eaten but he must compete with others 

 for food. Such a life brings constant perils, and it is probable 

 that few wild animals die a natural death. The mere power to 

 live on, therefore, dependent on these two conditions, that food 

 should be always present and enemies always absent, would be 

 of very little value. These giant tortoises may be said to have 

 tried evolution on such lines so far as it is possible for animals, 

 and the result is that they survive only on a few small islands. 

 From the great arenas of modern life they have disappeared. 



If, as I have tried to show, each successive step of evolution isolation 

 is due to some change in the environment, and generally to in- " t t *"g"f or 

 creased fierceness of competition with other species, then it is Natural 

 clear that mere isolation cannot, as Romanes thought, take the Selectlon 

 place of Natural Selection. It is quite true that Natural Selec- 

 tion is itself a form of isolation. The unfit are eliminated : death 

 parts them from the survivors by the most impassible of barriers. 

 But Natural Selection isolates in an altogether peculiar way. It 

 does not, like all other methods, set certain individuals apart 

 upon an island and leave them there to themselves. It brings 

 about a succession of isolations on a progressive system. It alone has 

 been able to accumulate variations and build up all the perfec- 

 tion of the plant and animal worlds. The achievements possible 

 for any other kind of isolation are very small. Supposing mem- 

 bers of some species to be put on various islands, on which the 



