Ill] 



OF WHALES AND TORTOISES 



173 



The horny plates of the tortoise grow, to begin with, a trifle faster 

 than the bony carapace below, and are consequently wrinkled into 

 folds. There is some evidence, at least in the young tortoises, that 

 these folds come once a year, which is as much as to say that there 

 is one season of the year when the growth-rates of bony and horny 

 carapace are especially discrepant. This would give an easy estimate 

 of age; but it is plainer in some species than in others, and it never 

 lasts for long. 



re m years 

 Fig. 42. Growth-rate (approximate) of blue and finner whales. 



The blue whale, or ' Sibbald's rorqual, largest of all animals, 

 grows to 100 ft. long or thereby, the females being a httle bigger 

 than the males. The mother goes with young eleven months. The 

 calf measures 22 to 25 ft. at birth, and weighs between three and 

 four tons; it is bom big, were it smaller it might lose heat too 

 quickly. It is weaned about nine months later, and is said to be 

 some 16 metres, or say 53 ft., long by then. It is believed to be 

 mature at two years old, by which time it is variously stated to be 

 60 or even 75 ft. long ; the modal size of pregnant females is about 

 80 ft. or rather more. How long, the whale takes to grow the 

 further 15 or 20 feet which bring it to its full size is not known; 

 but, even so far, the rapid growth and early maturity seem very 

 remarkable (Fig. 42). The Norwegian whalers give us statistics, 



