Transcriber’s notes: Italics is represented by _underscores_; boldface is represented by =equals signs=. Pages 1-400, Plates I-II, and some referenced footnotes are in Volume I. STUDIES IN THE THEORY OF DESCENT BY DR. AUGUST WEISMANN PROFESSOR IN THE UNIVERSITY OF FREIBURG _WITH NOTES AND ADDITIONS BY THE AUTHOR_ TRANSLATED AND EDITED, WITH NOTES, BY RAPHAEL MELDOLA, F.C.S. LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON WITH A PREFATORY NOTICE BY CHARLES DARWIN, LL.D., F.R.S. _Author of “The Origin of Species,” &c._ IN TWO VOLUMES VOL. II. WITH EIGHT COLOURED PLATES London: SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON CROWN BUILDINGS, 188, FLEET STREET 1882 [_All rights reserved_] I. LARVA AND IMAGO VARY IN STRUCTURE INDEPENDENTLY OF EACH OTHER. It would be meaningless to assert that the two stages above mentioned were _completely_ independent of one another. It is obvious that the amount of organic and living matter contained in the caterpillar determines the size of the butterfly, and that the quantity of organic matter in the egg must determine the size of the emergent larva. The assertion in the above heading refers only to the structure; but even for this it cannot be taken as signifying an absolute, but only a relative independence, which, however, certainly obtains in a very high degree. Although it is conceivable that every change of structure in the imago may entail a correlative change of structure in the larva, no such cases have as yet been proved; on the contrary, all facts indicate an almost complete independence of the two stages. It is quite different with cases of _indirect_ dependence, such, for example, as are brought about by ‘nurse-breeding.’ This phenomenon is almost completely absent in Lepidoptera, but is found in Diptera, and especially in Hymenoptera in every degree. The larvæ of ichneumons which live in other insects, require (not always, but in most instances) that the female imago should possess a sharp ovipositor, so that in this case also the structure and mode of life of the larva influences the perfect insect. This does not depend, however, on inherent laws of growth (correlation), but on the action of external influences, to which the organism endeavours to adapt itself by natural selection. I will now let the facts speak for themselves. It is shown by those species in which only one stage is di- or polymorphic that not every change in the one stage entails a corresponding change in the other. Thus, in all seasonally dimorphic species we find that the caterpillars of butterflies which are often widely different in the colour and marking of their successive generations are absolutely identical. On the other hand, many species can be adduced of which the larvæ are dimorphic whilst the imagines occur only in one form (compare the first and second essays in this volume).