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, 
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