128 THE AMERICAN NATURALIST [Vol. LVI 



variation is that we know almost nothing about the chem- 

 ical and physical factors which underly growth, hyper- 

 trophy, hyperplasia, metaplasia, or other changes in 

 somatic tissues due to changed internal relations or to 

 unusual environmental stimuli. If we only knew, for 

 instance, what happens in even as simple a case as when 

 epidermal cells develop into a callous in response to undue 

 pressure or friction, we might have a clue as to how, also, 

 constructive changes might occur in germ-cells; but we 

 have no such knowledge. 



Certain types of tissue-overgrowth^ in which there is 

 increase in the size of the tissue-elements {hypertrophy) 

 or in the number of such elements {hyperplasia) are 

 interesting in this connection. For example, increased 

 strain in bone leads to increased growth of bony tissue, 

 or excessive exercise leads to overdevelopment of certain 

 muscles. In such cases an increased demand on the nutri- 

 tive stream caused by unusual katabolism results in a 

 physiological hypertrophy. That is, an excessive syn- 

 thesis of certain types of proteins is set up. Does the 

 impetus to such extra synthesis extend also to the 

 related, though unstimulated, tissues? I know of no evi- 

 dence bearing directly on this point, although such phe- 

 nomena as compensatory overgrowth show that there are 

 influences at work outside the immediate tissue itself 

 which are instrumental in inducing the hypertrophy. 

 For example, if one of a pair of organs (lung, kidney, 

 testicle, thyroid) is lacking or is destroyed, the other en- 

 larges in a short time to the size and functional capacity 

 of the pair combined. There is considerable evidence, 

 particularly in the field of pathology, to show that under 

 ordinary conditions the tissue-elements exert a sort of 

 balanced reciprocal restraint, but disturb this and the 

 whole system is more or less deranged until a new equilib- 

 rium is established. Since in compensatory adjust- 

 ments the compensating organ is generally not in direct 

 connection with the one which is missing or disturbed, 

 it seems probable that the agent which incites the hyper- 



5 Cf. any General Pathology. 



