Fig. 20. _P. Agestis_ (_Alexis_, Scop.), Italian summer form. (The chief difference between figs. 19 and 20 lies on the under-side, which could not be here represented.) Fig. 21. _Polyommatus Phlæas_, winter form, from Sardinia; the German winter and summer generations are perfectly similar. Fig. 22. _P. Phlæas_, summer form, from Genoa. Fig. 23. _Pararga Ægeria_, from Freiburg, Baden. Fig. 24. _P. Meione_, southern climatic form of _Ægeria_ from Sardinia. END OF PART I. STUDIES IN THE THEORY OF DESCENT. Part II. ON THE FINAL CAUSES OF TRANSFORMATION. =I.= THE ORIGIN OF THE MARKINGS OF CATERPILLARS. INTRODUCTION. The general idea which has instigated the researches described in the present essay has already been expressed in the Preface, where it has also been explained why the markings of caterpillars, and especially those of the Sphinx-larvæ, were chosen for testing this idea. The task presented itself in the following form:--In order to test the idea referred to, it must be investigated whether all the forms of marking which occur in the Sphinx-larvæ can or cannot be traced to known transforming factors. That natural selection produces a large number of characters can be as little doubted as that many varying external influences can bring about changes in an organism by direct action. That these two transforming factors, together with their correlatively induced changes, are competent to produce _all_ characters, howsoever insignificant, has indeed been truly asserted, but has never yet been proved. The solution of the problem, however, appeared to me to depend particularly on this point. We are now no longer concerned in proving that a changing environment reacts upon the organism--this has already been shown--but we have to deal with the question whether _every change_ is the result of the action of the environment upon the organism. Were it possible to trace all the forms of markings which occur, to one of the known factors of species transformation, it could be thus shown that here at least an “innate power of development” was of no effect; were this not possible, _i.e._ did there remain residual markings which could not be explained, then the notion of an “innate principle of development” could not be at once entirely discountenanced.