DOMESTICATED ANIMALS 41 



Now, it is possible for it to make over an organ 

 which it does not need into one that it does need, 

 somewhat as our mothers used to transform a 

 coat which we did not need into a waistcoat or a 

 pair of trousers which we did need. 



The wings of birds were formed in this way out 

 of the fore legs of lizards. Birds have been de- 

 veloped from lizard-like reptiles. And in the 

 transformation of the scaly lizard into the feath- 

 ered bird the fore legs of the lizard went to form 

 the bird's wings. The bird's wing has the same 

 general architecture as the lizard's fore leg; hu- 

 merus, ulna and radius, carpal bones, and three 

 series of metacarpal bones. Two of the five toes 

 of the lizard have been lost in the bird's wing. 



But the transformation of superfluous organs 

 into useful organs is the exception. As a rule, or- 

 gans that are not needed go to waste. 



Now, it is a law that when organs are not used 

 they tend to disappear. Organs that do nothing 

 are not nourished, and hence tend to fade away. 

 Then, too, organs that are not used are not em- 

 phasized by Natural Selection. And if their use- 

 lessness continues long enough, they will not only 

 shrivel and decay, but will finally pass out of ex- 

 istence entirely. There are almost numberless 

 examples of extinction of this kind known to biolo- 

 gists. The disappearance of legs in snakes is an 

 instance. Snakes have come from lizards, and 

 originally walked on four legs. But in the strug- 

 gle for life they have found it of advantage to 



