THE ORIGIN AND THE EXTINCTION OF SPECIES 359 



evolved ; we also understand — and this is the point with wliich we 

 are here chiefly concerned — why these two ' excessive ' developments 

 should ultimately lead to the destruction of their possessors. For 

 a long period the Armadillos were able to save themselves from exter- 

 mination by increasing their bodily size and the strength of their 

 armour, and they thus saved themselves from persecution on the part 

 of beasts of prey with smaller and weaker teeth. But the predatory 

 animals followed suit and lengthened their teeth and increased their 

 bodily size, until ultimately even the strongest armour of the victim 

 aflTorded no efficient protection, and the mighty Glyptodonts were b}^ 

 degrees utterly exterminated. But then the death-knell of tlie 

 Machairodiis had also sounded, for he was so exactly adapted to this 

 one kind of diet that he could no longer overpower other victims and 

 feed on their flesh ; the sabre-teeth prevented him from tearing his 

 prey like other predatory animals, he could proljably only suck them. 



Even if this is a supposititious case, it serves to show that it was 

 not an internal principle of variation that caused the teeth of these 

 carnivores and the armour of their victims to increase so unlimitedly ; 

 it was the necessity of adaptation. They did not perish because 

 armour and teeth increased so excessively, but because neither of 

 these adaptations could be neutralized all at once, and small variations 

 were of no use to them in their final struggle for survival. 



In a certain sense we may say that simpler, more lowly organisms 

 are more capable of adaptation than those which are highly difleren- 

 tiated and adapted to specialized conditions in all parts of their 

 bodies, since from the former much that is new may arise in the 

 course of time, while very little and nothing very novel can spring 

 from the latter. From the simplest Protozoan the whole world of 

 unicellular organisms could arise, and also the much more diverse 

 Metazoa ; from the lower marine worms there could arise not only 

 many kinds of higher marine worms — the segmented worms or 

 Annelids — but also quite new groups of animals, the Arthropods and 

 the Vertebrates. It is hardly likely that a new class of animals 

 will evolve from our modern birds, because these are already so 

 perfectly adapted to their aerial life that tliey could hardly adapt 

 themselves to life on land or in the water sufficiently well to be able 

 to hold their own in regard to all the possibilities of life with the rest 

 of the dwellers on land or in water. We do indeed know of birds 

 which have returned entirely to a purely terrestrial life— the ostriches, 

 for instance— and of others Avhich have adapted themselves to 

 a purely aquatic life, such as the penguins, but these are small 

 groups of species, and are hardly likely to increase. On the contrary, 



