98 THE ORIGIN AND EVOLUTION OF LIFE 



of a centimetre, or i /lo, 000,000 of a millimetre, or i /io,ooo 

 of a micron — the unit of microscopic measurement. The elec- 

 trons released from atoms of matter are only 1/1800 of the 

 mass of the hydrogen atom, the lightest known to science, and 

 thus the mass of an electron would be only 1/18,000,000 of a 

 micron. 



These figures help us in some measure to conceive of the 

 chromatin as a microcosm made up of an almost unlimited 

 number of mutually acting, reacting, and interacting particles; 

 but while we know the heredity-chromatin to be the physical 

 basis of inheritance and the presiding genius of all phases of 

 development, we cannot form the slightest conception of the 

 mode in which the chromatin speck of the germ cell controls 

 the destinies of Sequoia gigantea and lays down all the laws of 

 its being for its long life period of five thousand years. 



In observing the trunk of "General Sherman" (Fig. 14), 

 the largest and oldest living thing known, one finds that an 

 active regeneration of the bark and woody layers is still in 

 progress, tending to heal scars caused by fire many centuries ago. 

 This regeneration is attributable to the action of the heredity- 

 chromatin in the plant tissues. 



We are equally ignorant as to how the chromatin responds 

 to the actions, reactions, and interactions of the body cells, of 

 the life environment, and of the physical environment, so as 

 to call forth a new adaptive character,^ unless it be through 

 some infinitely complex system of chemical messengers and 

 other catalytic agencies (p. 77). Yet in pursuing the history 

 of the evolution of life upon the earth we may constantly keep 

 before us our fundamental biologic law- that the causes of 

 evolution are to be sought within four complexes of energies, 

 which are partly visible and partly invisible, namely: 



1 Wilson, E. B., 1906, p. 434. - Osborn, H. F., 1912.2. 



