GENETIC TYPE AND THE END0CIUXES 25 



motor neurones in the lower portions of the spinal cord in 

 certain dog breeds and their hybrids which results in de- 

 generation and functional alteration of the hind extremities, 

 clearly suggesting the evolutionary loss and modification in 

 the posterior extremities of aquatic mammals. It is well 

 known that animals with rudimentary limbs, such as the 

 flightless birds with their undeveloped wings, have a much 

 reduced cervical enlargement of the spinal cord, while com- 

 pletely limbless animals, such as the snakes, show no enlarge- 

 ments of the cord in either the cervical or lumbosacral regions. 

 All the motor neurones which usually supply the muscles of 

 the extremities are absent from these spinal cords. It is 

 very probable that the neurones failed to appear or were 

 lost during development before the leg itself began to de- 

 generate. At any rate, it is quite certain that if through a 

 mutation the quota of motor neurones designed to supply the 

 muscles of the arm should fail to arise, the arm would neces- 

 sarily be motionless and eventually would wilt. Thus the 

 course of evolution in the modification or the loss of limbs 

 might have its origin in a mutation which is lethal for the 

 motor neurones in the cord supplying the limb, followed by 

 the resulting modifications and loss of muscles and other 

 tissues. Marine mammals, such as walruses, seals, porpoises 

 and whales, show different degrees of modifications, or more 

 exactly, of the degeneration and almost complete loss of the 

 posterior extremities. This is brought about by the loss of 

 many motor neurones and a reduction of the lumbosacral 

 enlargement of the spinal cord. It is difficult to imagine that 

 the entire change necessary to give this loss of nerve cells 

 in the spinal cord and these strangely altered posterior ex- 

 tremities could come about as a single mutation. It would 

 seem far more probable that a series of mutations or a certain 

 complexity of genie rearrangements was gradually evolved 

 in bringing about this modification of a walking extremity into 

 a flipper-like paddle. A careful study of the skeletons of 

 such animals shows evidence of achondroplasic reaction, and 

 other features suggest modified pituitary function. Here, 



