It requires but a slight alteration of the definition to make out of this “restricted” or “limited variability,” which is the necessary consequence of the physical nature of each species, a “fixed direction of variation” in the sense of a phyletic vital force. Instead of--the _Smerinthus_-larvæ show a tendency to produce red spots on the skin, it is only necessary to say--these larvæ tend to produce red borders to the oblique stripes. The latter statement would, however, be incorrect, since the red borders first arose by the coalescence of red spots through the action of natural selection. It is not even correct to say that _all_ the species of _Smerinthus_ show this tendency to produce spots, since this character does not seem to occur either in _S. Quercus_ or _S. Tremulæ_. The distinction between the two modes of conception will become clear if we ask, as an example, whether those _Chærocampa_-larvæ which do not at present possess eye-spots will subsequently acquire these markings, supposing that they maintain their existence on the earth for a sufficient period? The supporters of a “fixed direction of variation” would answer this question in the affirmative. Ocelli constitute a character which occurs in nearly all the species of the group--they are the goal towards which the phyletic force is urging, and which must sooner or later be reached by each member of the group. On the other hand, I cannot express so decidedly my own opinion, viz., that such complicated characters as the many-coloured oblique stripes or eye-spots are never the results of purely internal forces, but always arise by the action of natural selection, _i.e._ by the combination of such minute and simple variations as may present themselves. It may be replied that the formation of eye-spots in those species which are at present devoid of them, cannot indeed be considered impossible, but that they would only appear if the constitution of these species had a tendency to give rise to the production of darker spots on the edge of the subdorsal line, and if at the same time, the possession of eye-spots would be of use to the caterpillar under its special conditions of life.