6 APES AND MONKEYS 



in the structure of this bone, and have pointed out as im- 

 possible a common origin for man and ape ; but one fact 

 remains to be explained, and that is, that while these 

 appear to be fixed and constant characteristics of man and 



ape there are many exceptions known in human anatomy. 

 In the splendid collection of human spinal columns in the 

 museum of the Harvard Medical School are no less than 

 eighteen specimens of the human sacrum having six united 

 segments ; and I have found in the collections of various 

 museums a total of more than thirty others. These facts 

 show that this characteristic is not confined to the ape. 

 It is true that in some of these abnormal specimens there 

 remain five lumbar vertebrae. This seems to indicate that 

 this portion of the spinal column is the most susceptible 

 to variation. I have never seen an instance, however, of 

 variation in the sacrum of the chimpanzee. In this respect 

 he appears to be, in his structural type, more constant 

 than man. 



One reason why this bone is so formed in the ape is 

 this. At that point the greatest weight and strain are laid 

 upon the spinal column, and the crouching habit of the 

 animal has a tendency to depress the lowest lumbar verte- 

 bra between the points of the hip bones and thus arrest 

 its lateral movement. Since the flexure of this part is 

 lessened, the cartilage that lies between the two segments 

 becomes rigid and then ossifies. The erect posture of 

 man allows more play in the region of the loins, and hence 

 this motion prevents the two bones from uniting. 



Another bone that may be said to vary somewhat is the 

 sternum, or breastbone. It is the thin, soft bone to which 



