ORIGIN OF THE LIVING MACHINE: ADAPTATION 347 



development of the spinal column of the vertebrates during 

 the geological ages, which is disclosed by the fossils in the 

 rocks. When the vertebrates first appeared, apparently they 

 had no bones, but in their backs was a rather stiff rod which 

 gave them rigidity, this being represented by the rod in the 

 embryo which we have already learned to speak of as the 

 notochord (see page 286). Following along through the various 

 strata of rocks, which represent a progressive development of 

 vertebrates, we find that this rod in time became broken up 

 into short sections, a condition which adapted its possessor 

 very much better to an active life in the water. The short 

 sections, which became the vertebrae, enabled a lateral flex- 

 ing motion of the body which could not be brought about so 

 readily if there were only a stiff supporting rod in the back. 

 This broken series of bones, forming the vertebral column, 

 thus adapted the animal to its rapid motion in the water. 

 Later, when the vertebrates emerged from the water and as- 

 sumed a life on the land, the type of vertebrae adapted to life 

 in the water was no longer fitted for the condition in which 

 the animal now lived. The vertebrae were still retained, but 

 they acquired new connections with each other, a greater solidity 

 and a greater rigidity, so that the spinal column could now 

 support the body in the air. Further development of the land 

 animals into the birds was characterized by a further change in 

 the form of the vertebrae, which adapted the animal to life in 

 the air, and, moreover, the vertebrae were changed in another 

 fashion in the mammals which lived on the land. In all of these 

 series of changes, from the original unbroken rod of the back in 

 paleozoic times, to the complicated spinal column of the mam- 

 mal, we see a successive series of adaptations. The study of fos- 

 sils has made it possible to trace this series of changes in detail, 

 and our paleontologists have quite accurately pictured for us 

 the succession of changes that has produced this long series of 

 race variations, bringing about an adaptation of the race, first 

 to one condition of life and then to another, and finally ending 



