REPTILES AND AMPHIBIANS 1 27 



most of the tigers, and how, when the weaker 

 individuals had died out, only those tigers could 

 survive whose long teeth could bore through the 

 hard coat of the glyptodons. In this way the 

 balance would change from side to side for ever, 

 and it could never come to pass that "even the 

 stoutest armour would no longer protect the victim, 

 and the huge glyptodons be gradually extirpated."^ 

 Natural selection could only strengthen the armour of 

 the armadilloes if it was near the limit — that is to say, 

 if a slight thickening made it impenetrable for a large 

 number of tigers. If the latter could suddenly over- 

 come all the glyptodons, they must have lengthened 

 their teeth to the extent of two generations of 

 growth ; and that would be a leap upwards in the 

 evolution of the teeth, not a gradual advance. It is 

 only on the latter, not on leaps, that we must rely in 

 the modification of species by variation. We must, 

 therefore, trace the extinction of the glyptodons to 

 another cause. One might say, of course, that there 

 were limits in the nature of the animal that would 

 not allow the indefinite modification of an organ, and 

 so after a certain time the armour of the armadillo 

 could grow no thicker. Must there not be some 

 provision that trees do not grow up into the sky .■* 

 But we shall see in the eleventh chapter that this 

 phrase, however imposing it may seem, explains 



1 Weismann. It is further said that the tigers got the advantage 

 of the armadilloes by sticking their teeth into the unprotected neck. 

 But that can hardly have been the case, as then natural selection 

 would only have to form and strengthen a plate for the neck, and the 

 dorsal plate would remain as it was, not being subject to attack. 



