48 



cow and sheep. In the body of the whale where 

 hind- limbs would naturally be, there are found the 

 anatomical ruins of these organs in the form of a 

 few diminutive bones. The same thing is true in 

 the sirenians. In the Greenland whale there are 

 remnants of both femur and tibia in the region of 

 the atrophied hind-limbs. The snakes are limb- 

 less, but the pythons and boas have internal 

 remnants of hind-limbs, and sometimes even clawed 

 structures representing toes. The so-called 'glass- 

 snake ' or 'joint-snake' (which is really a limbless 

 lizard) has four complete internal limbs. Young 

 turtles, parrots, and whalebone whales have teeth, 

 but the adults of these animals are toothless. 

 Cows, sheep, deer, and other ruminants, never have 

 as adults any upper incisors, but these teeth are 

 found in the foetal stages of these animals just 

 under the gums. The female frog has rudimentary 

 male reproductive organs, and the male has cor- 

 responding vestiges of female organs. Similar 

 remnants of the reproductive structures exist in 

 many other animals. They represent stages in 

 the transition from the hermaphroditism of primi- 

 tive animals to the unisexuality of the higher 

 forms, the separation of the sex organs into those 

 of male and female having come about through the 

 decay of one set of structures in each individual. 



For reasons which it is not necessary to mention 

 here, biologists believe that insects all originated 

 from a common parental form, with two pairs of 

 wings and six legs. Insects all retain their original 

 allowance of legs, but in many species one or the 



