INTERNAL CAUSES OF VARIATION 



209 



from the same soil and the same atmosphere. The most nour- 

 ishing food and the most deadly poison are produced side by 

 side under identical external conditions. A man divides his 

 dinner with his dog : one portion simply nourishes the dog 

 and provides energy to watch sheep or perchance to kill them ; 

 the other results in strength to bless the ages, or perhaps in 

 crime to shock the world, each according not to the nature 

 of the food but to that of the animal that consumes it, and 

 to the support of whose peculiar energies it contributes. 



This kind of difference in living organisms is traceable to the 

 endowments of a single cell, the only material that passes 

 over from parent to offspring, and, regard it as we may, we 

 must see in this single bit of living matter all the potential 

 qualities of the race, all the differences between the corn and 

 the wheat, between the man and his dog. They are all there, 

 represented in some material way in the constitution of the 

 germ cell. There is thus a material basis to heredity. 



We must accept one horn or the other of the dilemma : 

 either conceive this single cell as directly endowed with all the 

 qualities of the race, defining its development, or else endow it 

 with the capacity to develop in this fashion or that according 

 to stimuli. But whence come the stimuli ? Certainly not alto- 

 gether from without, or the man and his dog would become 

 alike, if consuming the same kind of food ; and to assume that 

 the influences are internal is only to push the puzzle one step 

 farther away and to assume possibly an immaterial in place of a 

 material basis. 



The most simple and direct explanation of the phenomena of 

 inheritance and definite development is to consider the germinal 

 matter as consisting of units of some sort endowed with life 

 and the power of growth. This assumption of the physiological 

 unit is not so violent nor so different from other accepted scien- 

 tific assumptions as it at first may seem. In the non-living 

 world we assume the existence of the atom, whatever its ulti- 

 mate constitution, as a minute, indivisible, and indestructible 

 unit of matter. The association of some millions of like atoms 

 makes a measurable quantity of an element like gold, silver, 

 iron, chlorin, or sodium. 



