234 AGE, GROWTH, AND DEATH 



functions of life to go on. Other conditions are also 

 there, and to no one of them does the quality of life 

 properly attach itself. Of life units there is an ap- 

 palling array. The most estimable of them, in my 

 opinion, are the life units which were hypothetically 

 created by Charles Darwin in his theory of pangenesis. 

 He assumed that there were small particles (gem- 

 mules) thrown off from different portions of the body 

 circulating throughout the body, gathering sometimes 

 in the germ cells. These particles he assumed to take 

 up the qualities of the different parts of the body 

 from which they emanated, and by gathering together 

 in immense numbers in the germ cells to accomplish 

 the hereditary transmission. We know now that this 

 theory is not necessary, that it is not the correct 

 theory. But at the time that Darwin promulgated it, 

 it was a perfectly sound, defensible theory, a theory 

 which no one considering fairly the history of bio- 

 logical knowledge ought to criticise unfavourably. It 

 was a fine mental achievement, but I should like also 

 to add that of all the many theories of life units, this 

 of Darwin is the only one which seems to me intel- 

 lectually entirely respectable. Of supposed structural 

 life units there is a great variety. Besides the gem- 

 mules of Darwin, there were the physiological units 

 of Herbert Spencer. Professor Haeckel, the famous 

 German writer, has structural life units of his own 

 which he terms plastidules ; he gave his theory the 

 charming alliterative title of perigenesis of the plasti- 

 dules ; the rhythm of it must appeal to you all, though 



