12 GROWTH 



biological answer is heredity, which, as the word is commonly 

 used, means in plain language that our fathers were made that 

 way before us and we are made like them. A more correct an- 

 swer is that the particular cell from which each of us starts, us- 

 ing the word "us" in a broad biological sense to include every 

 living individual, has a particular kind of living stuff carrying 



Figure 8. Pharaoh's serpents. An "egg" similar to that 

 from which each "serpent" grew is shown at the left 

 of each of the serpents. The shape of the "egg" and 

 its composition, one of them containing a black mate- 

 rial in addition to mercury sulphocyanate, cause the 

 differences in color and form. 



on a particular series of chemical reactions and physical proc- 

 esses which work out into the characteristic structure of the 

 individual. 



We might use an analogy to make this point of view clearer. 

 Suppose we t?ke a small cone of mercury sulphocyanate and set 

 fire to the tip. The burning compound forms a long serpent- 

 like ash a hundred times or more the volume of the origi- 

 nal cone. This is the familiar firework called Pharaoh's serpent. 

 The growth of Pharaoh's serpent depends upon the particular 

 kind of material of which the cone is made. To have a cone of 

 charcoal, sulphur, or gunpowder will not do. It must be mer- 

 cury sulphocyanate. It depends also upon the chemical reactions 

 which this stuff carries on. Only burning (oxidation) will form 



