But although natural selection is the factor which has called into existence and perfected the three chief forms and certain of the subsidiary markings, in the repetition of the local character on the other segments, as well as in the formation of new elements of marking at the points of intersection of older characters now rudimentary, we can recognize a second factor which must be entirely innate in the organism, and which governs the uniformity of the bodily structure in such a manner that no part can become changed without exerting a certain action on the other parts--an innate law of growth (Darwin’s “correlation”).
Only once during the whole course of the investigations was it for an instant doubtful whether a phyletic vital force did not make itself apparent, viz., in the red spots accompanying the oblique stripes in several _Smerinthus_-larvæ. Closer analysis, however, enabled us to perceive most distinctly the wide gulf that separates “analogous variation” from the mystic phyletic vital force. Nothing further remains therefore for the action of this force in respect to the marking and colouring of the _Sphingidæ_, since several even of the subordinate markings can be traced to their causes, only the “dorsal spots” of our two native species of _Chærocampa_ having been referred to correlation without decided proof. From the temporary inability to explain satisfactorily such an insignificant detail, no one will, however, infer the existence of such a cumbrous power as a phyletic vital force.
The final result to which these investigations have led us is therefore the following:--The action of a phyletic vital force cannot be recognized in the marking and colouring of the _Sphingidæ_; the origination and perfection of these characters depend entirely on the known factors of natural selection and correlation.
ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.
In the previous essay I attempted to trace a whole group of apparently “purely morphological” characters to the action of known factors of transformation, to explain them completely by these factors, and in this manner I endeavoured to exclude the operation of an internal power inciting change (phyletic vital force).