358 THE EVOLUTION THEORY 



and minus fluctuations hy means of" which personal selection can 

 operate. 



But of course it requires a certain amount of time for this, and 

 in the fact that this time is often not available lies, I think, the 

 reason why excessive differentiations havfe often led to the extinction 

 of a species, not because the increase of the excessive organ must 

 go on irresistibly, but because changes in the conditions have made 

 the exuberant organ inappropriate, and it could not degenerate quickly 

 enough to save the species from extinction. 



Brandes has recently given a beautiful illustration of this by 

 associating the existence of the remarkable sabre-toothed tigers 

 (Machairodiis) with enormously long canine teeth, which lived in the 

 Diluvial period in South America, with the gigantic Armadillos which 

 lived there at the same time, whose bony armature two yards in 

 height now excites our admiration. He rightly points out that the 

 dentition of Machalrodus iieogoius is by no means a typically perfect 

 dentition for a beast of prey, like that of the Indian tiger or the lion ; 

 as far as incisors and molars are concerned it was much less effective 

 than that of these predatory animals, and the great length of the 

 dagger-like flattened canines, which protruded far bej^ond the mouth, 

 entirely prevented the bringing together of the teeth of the ujiper and 

 lower jaw after the fashion of a pair of pincers. He rightly infers 

 from this that this dentition was adapted to a specialized mode of 

 nutrition, and he regards the great mailed Armadillos, such as the 

 three-yards-long heavy Glyptodont of the Pampas, as the victims into 

 which they were wont to thrust their sabre-teeth in the region of the 

 unprotected neck, and thus to master the almost invulnerable creature, 

 which was invincible as far as all other predatory animals were con- 

 cerned. Thus the remarkable dentition is explained on the one hand, 

 and on the other the amazing extent and hardness of the victim's 

 coat of mail. Thus, too, we can understand why there should have 

 been at that time a whole series of cat-like animals with sabre-like 

 teeth, in which the length and sharpness of the teeth increased with 

 the bodily size, for these predatory animals corresponded to a whole 

 series of Armadillos, whose size was increasing, as was also the strength 

 of their armour. 



Of course this interpretation is hypothetical, but it contains 

 much internal probability, so that it may be taken as a good illustra- 

 tion of the reciprocal increase of adaptations between two animal 

 groups. We understand now why, on the one hand, this colossal 

 tortoise-like armour should have developed in a mammal, and, on the 

 other hand, why these enormously long sabre-teeth should have been 



