Similarity is thus a quantitative property. The first case is the most obvious offense but perhaps the least troublesome. Nonetheless, the author's ideas are likely to be clear since arguments against common ancestry are presented explicitly. The final case occurs most frequently and is the most subtle and therefore most troublesome. In this case, the two meanings of homology seem to overlap, and it is almost inevitable that the thinking of author and reader alike will be intrusively distorted as follows.
Similarity is relatively straightforward to document.
A similarity, then, can become a fully documented, simple fact. On the other hand, a common evolutionary origin must usually remain a hypothesis, supported by a set of arguments that might include sequence or three-dimensional similarity. Not all similarity connotes homology but that can be easily over looked if similarities are called homologies. Homology among similar structures is a hypothesis that may be correct or mistaken, but a similarity itself is a fact, however it is interpreted. We believe that the concepts of evolutionary homology and sequence or three-dimensional similarity can be kept distinct only if they are referred to with different words.