The Structural material of the wheels discussed in the passage would be similar to that of 

In the past evolutionary biologists contemplating the absence of wheels in nature agreed that the explanation was not undesirability; wheels would be good for animals, just as they are for us. Animals were prevented from evolving wheels the biologists reasoned, by the following dilemma Living cells in an animal's body are connections at the first revolution making living wheels impracticable However there is a flaw in the argument that the evolution of wheeled animals was thwarted by the insoluble joint problem. The theory fails to explain why animals have not for arteries and nerves. Countless animals, 'including us, beer external structures without blood supply or nerves for example, our hair and fingernails, or the scales, claws, and horns of other animals. why have rats not evolved bony wheels, similar to roller skates ? Paws might be more useful than wheels in some situations, but cats claws are retractable why not retractable wheels ? We thus arrive at the serious biological paradox flippantly termed the RRR dilemma; nature's failure to produce rats with retractable roller skates.