254 CHARLES R. STOCKARD 



similar cytological complexes show more nearly similar growth 

 and tissue reactions than do random groups of individuals derived 

 from non-related parentage. 



In the old individual with but little normal growth still in exist- 

 ence, there can be, on the basis of my interpretation, but slight 

 inhibition to any regenerative process that might be set up. 

 Such animals naturally on account of their old condition usually 

 regenerate very slowly, but following continued trauma, active 

 regenerative growths are frequently begun, and not being under 

 the inhibiting control of any other active growth processes, this 

 regeneration attains an excessive, distorted, and malignant con- 

 dition. 



All very old animals no doubt experience a considerable amount 

 of trauma, and if they lived long enough almost all of them might 

 possess some cancerous growths. The truth of this statement is 

 well illustrated by comparing the frequency of reported cancer in 

 rats and mice with similar growths in guinea-pigs, all constantly 

 used laboratory animals. Rats are very old after three years of 

 life, and actually at two years old may properly be compared, 

 according to Donaldson ('15), with a man at sixty. Mice attain 

 old age even earlier, and at two years are very old. This being 

 the case, it frequently happens that the rats and mice used in 

 laboratories have actually become old individuals, having been 

 kept by the breeders and the laboratory for as long as two years. 

 Cancerous growths are common in these animals. 



The guinea-pig under favorable conditions does not become 

 old until it has lived for about five years, and we have frequently 

 kept these animals for more than seven years; at this age, how- 

 ever, they are extremely old. Thus, as a rule, the guinea-pigs 

 used in laboratories are really young individuals, generally less 

 than three or four years old. Consequently, cancerous growths 

 are said to be uncommon among these animals. However, among 

 the old individuals in our stock a considerable percentage of can- 

 cerous ones have occurred. So it might be inferred that if as 

 great a number of really old guinea-pigs were observed as of old 

 rats and mice, cancer might be found to be almost as common 

 among guinea-pigs as among rats and mice. And finally it may 



