The following questions now present themselves: Have the markings of caterpillars any biological value, or are they in a measure only sports of nature? Can they be considered as partially or entirely the result of natural selection, or has this agency had no share in their production? The problem here offers itself more distinctly than in any other group of living forms, because it presents an alternative without a third possibility. In other words, if it is not possible to show that larval markings have a distinct biological significance, there remains only for their explanation the assumption of a phyletic force, since the direct action of the environment is insufficient to account for such regularity of development throughout a series of forms. The explanation by sexual selection is excluded _ab initio_, since we are here concerned with larvæ, and not with reproductive forms.[141] The biological significance of marking--if such significance it possess--will be most easily investigated by examining whether species with similar markings have any conditions of life in common which would permit of any possible inference as to the significance of the markings. Among the _Sphingidæ_ we find four chief forms of marking; (1) complete absence of all marking; (2) longitudinal stripes; either a simple subdorsal or this together with a spiracular and dorsal line; (3) oblique stripes; (4) eye-spots and ring-spots, single, paired, or in complete rows. Now if we consider in which species these four kinds of marking are of general occurrence, not only in the small group of the _Sphingidæ_ but in the whole order Lepidoptera, we shall arrive at the following results:-- 1. _Complete absence of marking_, so common in the larvæ of other insects, such as the Coleoptera, is but seldom found among Lepidopterous caterpillars.