The Making of Species 
extend this list. It must suffice that almost 
every investigator of the phenomena of inherit- 
ance believes in these units, and calls them 
by a different name. Moreover, each clothes 
them with characteristics according to his taste 
or the fertility of his imagination. 
These units behave in such a way as to sug- 
gest to us an analogy between them and the 
chemical molecules. The sexual act would appear 
to resemble a chemical synthesis in some respects. 
One of the most remarkable phenomena of 
chemistry is that of isomerism. It not in- 
frequently happens that two very dissimilar 
substances are found, upon analysis, to have the 
same chemical composition, that is to say, their 
molecules are found to be composed of the same 
kind of atoms and the same number of these. 
Thus chemists are compelled to believe that the 
properties of a molecule are dependent, not only 
on the nature of the atoms which compose it, but 
also on the arrangement of these within the 
molecule. To take aconcrete example: Analysis 
shows that both alcohol and ether are represented 
by the chemical formula C,H,O. In other words, 
the molecule of each of these compounds is made 
up of two atoms of the element Carbon, six of 
the element Hydrogen, and one of the element 
Oxygen. Now, every chemical atom possesses the 
property which chemists term valency, in other 
words, the number of other atoms with which 
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