Page:George Archdall Reid 1896 The present evolution of man.djvu/59

Rh of the first and ending with those of the last, and therefore in his development he presents a fleeting resemblance to each ancestor in turn.

It follows, therefore, that what is known as atavism is nothing other than a failure to recapitulate in the ontogeny the last stages of the phylogeny; i.e. it is an arrest of development, the individual halting at a stage reached by a remote ancestor, and developing no farther. A race may differ from its ancestry in three ways: (1) through evolution as a result of selection; (2) through evolution as a result of reversed selection; and (3) through retrogression occurring in the absence of selection. As regards traits evolved under the influence of selection, atavism is a simple arrest of development; that is, the ancestral form is approximated to, because the last stages of the phylogeny are omitted in the ontogeny. As regards traits evolved under the influence of reversed selection, atavism is also an arrest of development; the ancestral form reappears because there is an omission in the last stages of the ontogeny to retrace steps previously made, as was done in the phylogeny. As regards traits suppressed through retrogression, i.e. through a lapsing by the race of the last steps of the evolution, there can of course be no atavism; for instance, if the lost toes of the horse have disappeared through retrogression, i.e. through a return, in the absence of selection, to a very remote ancestral condition when they did not exist, then their reappearance in the modern horse would be an instance of evolution, not of retrogression, since it would be a return from a more ancient to a much more modern condition; on the other hand, if the toes have disappeared as a result of reversed selection, the embryo should exhibit them, and they should be present in cases of atavism.

It is now easy to understand why all races tend to