In Eugenie Scott's review of my book How Science Works: Evolution (Nature 465, 164; 2010), she perpetuates the common error of confusing the definition of the biological term 'homology' with its interpretation.

The word was invented in 1843 by anatomist Richard Owen to mean “the same organ in different animals under every variety of form and function”. But Owen did not believe in evolution and interpreted the observation of homologies as the preferences of a supernatural agent.

Modern biologists define homology differently, to mean the occurrence of similar traits in different organisms that are evolutionarily related by descent. To cite the modern definition as part of the evidence for evolution is a classic example of a circular argument, as pointed out by evolutionary biologist Mark Ridley in his textbook Evolution (Blackwell, 2003).

The solution is to avoid using 'homology' when discussing the evidence for evolution, and instead use 'similarity', the meaning of which is intuitively obvious but implies no particular interpretation. Homology can then be used to describe one result of evolution.

As I indicate in my book, the strongest evidence for evolution is the widespread observation of similarities at all levels of biological observation — from the anatomical to the molecular.