Thus in these last four essays much that appears heterogeneous will be found in close association, viz. scientific details and general philosophical ideas. In truth, however, these are most intimately connected, and the one cannot dispense with the other. As the detailed investigations of the three essays find their highest value in the general considerations of the fourth, and were indeed only possible by constantly keeping this end in view, so the general conclusions could only grow out of the results of the special investigations as out of a solid foundation. Had the new materials here brought together been already known, the reader would certainly have been spared the trouble of going into the details of special scientific research. But as matters stood it was indispensable that the facts should be examined into and established even down to the most trifling details. The essay “On the Origin of the Markings of Caterpillars” especially, had obviously to commence with the sifting and compilation of extensive morphological materials. AUGUST WEISMANN. _Freiburg in Baden, November, 1881._ CONTENTS. =Part I.= ON THE SEASONAL DIMORPHISM OF BUTTERFLIES. I. _The Origin and Significance of Seasonal Dimorphism_, p. 1. Historical preliminaries, 1. Does not occur in other orders of insects, 4. Beginning of experimental investigation, 5. Lepidopterous foes, 7. First experiments with _Araschnia Levana_, 10. Experiments with _Pieris Napi_, 13. Discussion of results, 17. Origination of _Prorsa_ from _Levana_, 19. Theoretical considerations, 23. The case of _Papilio Ajax_, 30. Experiments with _Pieris Napi var. Bryoniæ_, 39. The summer generations of seasonally dimorphic butterflies the more variable, 42. II. _Seasonal Dimorphism and Climatic Variation_, p. 45. Distinction between climatic and local varieties, 45. The case of _Euchloe Belia_ and its varieties, 47. The case of _Polyommatus Phlæas_, 49. The case of _Plebeius Agestis_, 50. III. _Nature of the Causes producing Climatic Varieties_, p. 52. Seasonal dimorphism of the same nature as climatic variation, 52. How does climatic change influence the markings of a butterfly? 52. The cause of this to be found in temperature, 54. Part played by the organism itself, 58. Analogous seasonal dimorphism in _Pierinæ_, 60. The part played by sexual selection, 62. IV. _Why all Polygoneutic Species are not Seasonally Dimorphic_, p. 63. Homochronic heredity, 63. Caterpillars, pupæ and eggs of summer and winter generations of seasonally dimorphic butterflies alike, 64. The law of cyclical heredity, 65. Climatic variation of _Pararga Ægeria_, 68. Continuous as distinguished from alternating heredity, 68. Return from dimorphism to monomorphism, 70. Seasonally dimorphic species hibernate as pupæ, 71. Retrogressive disturbance of winter generations, 72. The case of _Plebeius Amyntas_, 75. V. _On Alternation of Generations_, p. 80.