THE NATURAL HISTORY OF MAN 65 



of an animal to replace lost parts. The power of regeneration 

 in an animal, in general, is found to be in inverse ratio to the 

 degree of specialization, or differentiation, which it exhibits. 

 For example, among the lower vertebrates, the amphibia are 

 able to regenerate entire limbs, whereas in the more highly 

 specialized animals such as man this power of regeneration 

 is present only in certain tissues, notably the connective and 

 epithelial, by the agency of which wounds are healed. Cancer 

 is generally regarded as a localized lawless and unrestrained 

 growth of epithelium, the cells having become parasites and 

 attacked the host. The only cure thus far discovered is an 

 early destruction or removal of the abnormal parasitic cells. 

 The causation of cancer apparently lies in the disturbed bal- 

 ance of the forces stimulating and restraining growth in the 

 affected cells and is probably essentially a faulty cellular 

 chemistry. 



The child sits up by the sixth month, creeps by the tenth 

 month, and walks by the fifteenth month, thus passing from a 

 quadrupedal gait to the erect position in a few months, an 

 accomplishment which in evolution may have occupied ages. 

 All parts of the child do not grow at the same rate and, as 

 a result, the body proportions continually change during 

 growth (Fig. 24). Even the shaping of the features is due to 

 the different rates of growth of the various parts of the face. 

 Comparing the condition at birth with that of maturity it is 

 found that while the head doubles in height, the body increases 

 three times, the upper limbs four times, and the lower limbs 

 five times. These changes in the body proportions are well 

 shown if, with the adult proportions, we compare the pro- 

 portions of the child when expanded to the height of the adult. 

 Such a comparison will show, as has been previously noted, 

 that the head of the adult is relatively smaller, the arms 

 shorter, and the legs longer (Fig. 24). 



Likewise it is known that various organs grow at different 



