SYMMETRY (avv—furpov). In its general acceptation this word means a just and har monious proportionment of parts to one another and to their whole ; in anatomy, how ever, it has a different and more restricted meaning. With its anatomical signification alone I have now to deal, and that may be defined as follows : — Symmetry is a word used to express an idea that would be more correctly represented by a verb than a noun, for it is the idea of not a thing but a fact— the fact, namely, that one half of an animal is usually an exact reversed copy of the other— the right side is a reversed copy, or repetition, of the left. To this there are numerous ex ceptions, even in the human subject ; of which hereafter.
That unreversed serial copying or repetition which is observable, for instance, between the scapular and pelvic limbs, is enunciated by the analogous expression serial homology. The point on which a distinction may be made between symmetry and homology, is that of the reversing of the copy or repetition as characteristic of the latter. This characteristic seems to impress one with the notion that the two halves are parts of a whole, whereas an unreversed serial recurrence of similar parts inclines one to accord a kind of separate in dividuality to each repetition. A clear dis
tinction between these two styles of repetition ought undoubtedly to be firmly impressed and maintained on the mind. I have, however, for want of a convenient inflection of the word under consideration, at the risk of some confusion, been long accustomed to use the expression lateral homology in reference to symmetrical repetition—and in that sense I shall have to use it in this article.
Whether the word symmetry should be applied to that antero-posterior repetition which is met with in caudal vertebrm of fishes, for instance, is not yet determined by usage, and it will be sufficient for me hereafter simply to make my remarks upon the apparent exist ence of it. In so doing I shall use the ex pression antero-posterior homology in a sense precisely parallel to that of lateral homology.