If we wish to give an account of the atomic constitution of the aromatic compounds, we are bound to explain the following facts:
1) All aromatic compounds, even the most simple, are relatively richer in carbon than the corresponding compounds in the class of fatty bodies.
2) Among the aromatic compounds, as well as among the fatty bodies, a large number of homologous substances exist.
3) The most simple aromatic compounds contain at least six atoms of carbon.
4) All the derivatives of aromatic substances exhibit a certain family likeness; they all belong to the group of 'Aromatic compounds'. In cases where more vigorous reactions take place, a portion of the carbon is often eliminated, but the chief product contains at least six atoms of carbon These facts justify the supposition that all aromatic compounds contain a common group, or, we may say, a common nucleus consisting of six atoms of carbon. Within this nucleus a more intimate combination of the carbon atoms takes place; they are more compactly placed together, and this is the cause of the aromatic bodies being relatively rich in carbon. Other carbon atoms can be joined to this nucleus in the same way, and according to the same law, as in the case of the group of fatty bodies, and in this way the existence of homologous compounds is explained.

The whole aim of comparative anatomy is to discover what structures are homologous.