LOSS OF CALCAREOUS ARMOUR IN MAMMALS 207 



of it in lime rocks, must have, in course of time, caused something 

 like a lime famine in the process of multiplication of animals, hence 

 the nudity of a number of animals whose ancestors at one time must 

 have been more or less encased in calcareous armour. 



Lime salts must have been abundant everywhere, not only in 

 water plants and animals, but also in land plants and animals. 

 Lime was largely needed not only for building up of the exo- 

 skeletons, but also for building up the 72</0- skeletons of those 

 extinct monsters, the thigh-bone of some of which was taller 

 than a six-foot man, and proportionately massive. 



It is therefore not to be wondered at that animals, multiplying 

 indefinitely, may have eventually reached a period in which there 

 was an insufficient supply of lime to supply all demands. 



The endo-skeleton, if not sufficiently supplied with lime, would 

 become rachitic. The animal with a soft endo-skeleton would 

 collapse, and be of no use in the battle of life. So it was the 

 exo-skeleton that had to dispense with the calcareous stiffening, 

 and the animal world may then have begun to experience the 

 pathological phenomenon of animals with a rachitic skin \ cer- 

 tainly a great advantage where activity had become a factor in 

 the struggle for life. 



Even when land mammals had lost their exo-skeleton, they 

 must have continued to use up incredible amounts of carbonate 

 of lime for building up their endo-skeletons ; and when we read of 

 the swarms of ruminants which blackened the plains of Africa 

 and North America, we begin to realise the amount of this lime 

 salt that must have been hoarded in their endo-skeletons. But 

 this salt of lime such countless ruminants could evidently obtain 

 in sufficient quantity from the grasses and trees on which they 

 fed. 



And this very carbonate of lime which nourished the grasses 



