Biological Isomerism 



the material for the formation of the butterfly is 

 already stored up in the pupa. The unit char- 

 acters, or their precursors, are all there, and they 

 take one form or another according to the stimulus 

 applied. 



Phenomena of this kind can, we think, be 

 accounted for only on the assumption that the 

 unit characters affected are each developed from 

 a definite portion of the fertilised egg, that each 

 of these portions, these precursors of the unit 

 characters, is, like a chemical molecule, made up 

 of a number of particles, and that upon the 

 arrangement of these particles in its precursor 

 in the egg depends the form that the unit char- 

 acter derived from it will take. One arrange- 

 ment of these particles gives rise to one form of 

 unit character, while another arrangement will 

 give rise to a totally different form of unit 

 character. 



Thus, some organisms seem to display a bio- 

 logical isomerism akin to chemical isomerism, 

 save that the particles which in organisms 

 take the place of chemical atoms are infinitely 

 more complex. 



In other words, the precursors in the fertilised 

 egg of each of these unit characters behave in 

 some respects like chemical molecules. 



In order to avoid the manufacture of fresh 

 terms we may speak figuratively of the germ 

 cells as being composed of biological molecules, 



'57 



