198 



The Journal of Heredity 



the organism of a set of conditions which 

 permit of, or even bring about, the 

 optimum activity of the various cells 

 and organs of the body under varying 

 conditions of activity. The organism 

 has been made more efficient. 



There are other consequences, how- 

 ever, which should be considered more 

 at length. These are, as I shall attempt 

 to show, (1) the provision of a more 

 favorable environment for the cells 

 than exists outside the organism, per- 

 mitting differentiation, (2) the possi- 

 bility of an increased length of life of 

 individual cells withovit undergoing divi- 

 sion, and (3) the limitation of the pos- 

 sible variations occurring in the lifetime 

 of the individual. Wc may take up the 

 consideration of these various conse- 

 quences in order. 



1 . Differentiation is possible in a high 

 degree only when the tissues are pro- 

 tected from external influences. The 

 action of enz^Tnes upon food could not 

 reach its highest point of efficiency if 

 sea water passed through the digestive 

 cavity without restriction. The case 

 is still clearer in terrestrial forms. 

 No such tissue as the nerve tissue of 

 higher animals could be developed if it 

 were continually exposed to the drying 

 action of the atmosphere. The list 

 could be considerably extended. 



EFFECTS OF DIFFERENTIATION 



But differentiation is accompanied by 

 certain limitations of cell growth which 

 have led some biologists to seek in it the 

 origin of death. The conditions which 

 have made differentiation possible 

 would, on the basis of such a view, also 

 be the conditions which have made the 

 immortality of the Metazoa im]X)S.sible. 



There are certain objections to such 

 a view which have apparently been over- 

 looked. There is no inherent reason 

 why the loss of any one general char- 

 acteristic property of a cell should be 

 accompanied by the loss of the ])ower 

 to divide. Gland cells in general may 

 have lost the power of contractility, 

 but they still retain sufficient i)ower of 

 cell division to give rise to tumor 

 masses which may invade every region 

 of the body. 



Experiments on growing tissues, ])ar- 



ticularly plant tissues, in artificial cul- 

 ture media have shown that the cells, 

 no matter how specialized, revert to 

 the embryonic type — dedifferentiate as 

 Child puts it — and from this embryonic 

 type go on to reprodvice the whole new 

 organism. Cells can be modified if the 

 conditions in which they develop can be 

 modified. 



Nerve cells are considered to be 

 highly specialized cells, and their limit 

 of division is reached sooner in onto- 

 genetic development than is the case for 

 other cells. Yet the number of nerve 

 cells has continually increased as succes- 

 sively higher types of animals have 

 appeared. There are more nerve cells 

 in a monkey than in a frog, and more in 

 the htuTian subject than in the monkey. 

 It seems illogical to regard as the reason 

 for the onset of death of the cell, the 

 limitation of cell division in a tissue 

 which is manifesting greater nimibers 

 of cell divisions as evolution proceeds. 



2. The individual cells in a higher 

 organism in which all the mechanisms 

 of regulation are highly developed may 

 live vastly longer without undergoing 

 cell division than any cells in such a 

 long-lived form as the giant trees of 

 California. The nerve cells in which 

 originate the impulses leading to the 

 respiratory movements do not, so far as 

 I am aware, divide after birth; but from 

 the moment of the first respiratory 

 movement to the last breath man draws 

 at the close of his threescore years and 

 ten, these cells continue to function 

 under all conditions of rest or activity. 



3. The limitation of the magnitude 

 of possible changes during the life of 

 any one organism is a direct conse- 

 quence of the de\'eloi)ment of a high 

 degree of regulation. This limitation 

 covers the chemical constitution of the 

 organism, since variations beyond cer- 

 tain narrow limits are incompatible 

 with continued existence. The higher 

 organism either maintains its constancy 

 of conditions under all variations of the 

 external environment or dies when an 

 adjustment is no longer possible. There 

 is also a limitation of structural changes. 

 Unrestricted growth of any organ or 

 tissue in the body interferes with the 

 general equilibrium; the etlicienc}' of the 



