8o THE PHYSICAL KINSHIP 



Mesozoic Era, and the sipping insects (butter- 

 flies) in the Cenozoic. The flower-loving insects 

 (the bees and butterflies) came into the world at 

 the same time as did the flowers. The wings oi 

 insects may be modifications of the gills used by 

 insect young in respiration during their aquatic 

 existence. They are, hence, very different in 

 origin from the wings of birds, which are the 

 modified fore-legs of reptiles. 



The most important class of animals arising out 

 of the worms, on account of their distinguished 

 offspring, were the hypothetical cord animals. 

 The only existing species allied to these animals 

 is the amphioxus, a strange, unpromising-looking 

 creature, half worm and half fish, found in the 

 beach sands of many seas. It has white blood and 

 a tubular heart. It is without either head or 

 limbs, and looks very much like a long semi- 

 transparent leaf, tapering at both ends. But it 

 has two unmistakable prophecies of the vertebrate 

 anatomy: a cartilaginous rod, pointed at both 

 ends, extending along the back, and above this, 

 and parallel to it, a cord of nerve matter. These 

 are the same positions occupied by the spinal 

 column and spinal cord in all true vertebrates. 

 That the amphioxus is a genuine relative of the 

 ancestor of the vertebrates is also shown by the 

 fact that these simple forms of column and cord 

 possessed by amphioxus are precisely the forms 

 assumed by the spinal column and spinal cord in 

 the embryos of all vertebrates, including man. 



From these quasi-vertebrates developed the fishes 



