Studies in the Theory of Descent, Volume I. Weismann August
requires no explanation, now go too far towards the other extreme. The term “larval reproduction” is an expression, but no explanation; we have therefore to attempt to find out the true interpretation, but whether the one which I have given is correct must be judged of by others.
These four essays lead up to a fifth and concluding one “On the Mechanical Conception of Nature.” Whilst the results obtained are here summed up, it is attempted to form them into a philosophical conception of Nature and of the Universe. It will be thought by many that this should have been left to professed philosophers, and I readily admit that I made this attempt with some misgiving. Two considerations, however, induced me to express here my own views. The first was that the facts of science are frequently misunderstood, or at any rate not estimated at their true value, by philosophers;2 the second consideration was, that even certain naturalists and certainly very many non-naturalists, turn distrustfully from the results of science, because they fear that these would infallibly lead to a view of the Universe which is to them unacceptable, viz. the materialistic view. With regard to the former I wished to show that the views of the development of organic Nature inaugurated by Darwin and defended in this work are certainly correctly designated mechanical; with reference to the latter I wished to prove that such a mechanical conception of the organic world and of Nature in general, by no means leads merely to one single philosophical conception of Nature, viz. to Materialism, but that on the contrary it rather admits of legitimate development in a quite different manner.
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.
Freiburg in Baden,
November, 1881.
Part I. ON THE SEASONAL DIMORPHISM OF BUTTERFLIES
I. The Origin and Significance of Seasonal Dimorphism
The phenomena here about to be subjected to a closer investigation have been known for a long period of time. About the year 1830 it was shown that the two forms of a butterfly (Araschnia) which had till that time been regarded as distinct, in spite of their different colouring and marking really belonged to the same species, the two forms of this dimorphic species not appearing simultaneously but at different seasons of the year, the one in early spring, the other in summer. To this phenomenon the term “seasonal dimorphism” was subsequently applied by Mr. A. R. Wallace, an expression of which the heterogeneous composition may arouse the horror of the philologist, but, as it is as concise and intelligible as possible, I propose to retain it in the present work.
The species of Araschnia through which the discovery of seasonal dimorphism was made, formerly bore the two specific names A. Levana and A. Prorsa. The latter is the summer and the former the winter form, the difference between the two being, to the uninitiated, so great that it is difficult to believe in their relationship. A. Levana (Figs. 1 and 2, Plate I.) is of a golden brown colour with black spots and dashes, while A. Prorsa (Figs. 5 and 6, Plate I.) is deep black with a broad white interrupted band across both wings. Notwithstanding this difference, it is an undoubted fact that both forms are merely the winter and summer generations of the same species. I have myself frequently bred the variety Prorsa from the eggs of Levana, and vice versâ.
Since the discovery of this last fact a considerable number of similar cases have been established. Thus P. C. Zeller3 showed, by experiments made under confinement, that two butterflies belonging to the family of the ‘Blues,’ differing greatly in colour and marking, and especially in size, which had formerly been distinguished as Plebeius (Lycæna) Polysperchon and P. Amyntas, were merely winter and summer generations of the same species; and that excellent Lepidopterist, Dr. Staudinger, proved the same4 with species belonging to the family of the ‘Whites,’ Euchloe Belia Esp. and E. Ausonia Hüb., which are found in the Mediterranean countries.
The instances are not numerous, however, in which the difference between the winter and summer forms of a species is so great as to cause them to be treated of in systematic work as distinct species. I know of only five of these cases. Lesser differences, having the systematic value of varieties, occur much more frequently. Thus, for instance, seasonal dimorphism has been proved to exist among many of our commonest butterflies belonging to the family of the ‘Whites,’ but the difference in their colour and marking can only be detected after some attention; while with other species, as for instance with the commonest of our small ‘Blues,’ Plebeius Alexis (= Icarus, Rott.), the difference is so slight that even the initiated must examine closely in order to recognize it. Indeed whole series of species might easily be grouped so as to show the transition from complete similarity of both generations, through scarcely perceptible differences, to divergence to the extent of varieties, and finally to that of species.
Nor are the instances of lesser differences between the two generations very numerous. Among the European diurnal Lepidoptera I know of about twelve cases, although closer observation in this direction may possibly lead to further discoveries.5 Seasonal dimorphism occurs also in moths, although I am not in a position to make a more precise statement on this subject,6 as my own observations refer only to butterflies.
That other orders of insects do not present the same phenomenon depends essentially upon the fact that most of them produce only one generation in the year; but amongst the remaining orders there occur indeed changes of form which, although not capable of being regarded as pure seasonal dimorphism, may well have been produced in part by the same causes, as the subsequent investigation on the relation of seasonal dimorphism to alternation of generation and heterogenesis will more fully prove.
Now what are these causes?
Some years ago, when I imparted to a lepidopterist my intention of investigating the origin of this enigmatical dimorphism, in the hope of profiting for my inquiry from his large experience, I received the half-provoking reply: “But there is nothing to investigate: it is simply the specific character of this insect to appear in two forms; these two forms alternate with each other in regular succession according to a fixed law of Nature, and with this we must be satisfied.” From his point of view the position was right; according to the old doctrine of species no question ought to be asked as to the causes of such phenomena in particular. I would not, however, allow myself to be thus discouraged, but undertook a series of investigations, the results of which I here submit to the reader.
The first conjecture was, that the differences in the imago might perhaps be of a secondary nature, and have their origin in the differences of the caterpillar, especially with those species which grow up during the spring or autumn and feed on different plants, thus assimilating different chemical substances, which might induce different deposits of colour in the wings of the perfect insect. This latter hypothesis was readily confuted by the fact, that the most strongly marked of the dimorphic species, A. Levana, fed exclusively on Urtica major. The caterpillar of this species certainly exhibits a well-defined dimorphism, but it is not seasonal dimorphism: the two forms do not alternate with each other, but appear mixed in every brood.
I
I am aware that this certainly cannot be said of philosophers like Lotze or Herbert Spencer; but these are at the same time both naturalists and philosophers.
“Die Arten der Lepidopteren-Gattung
[Eng. ed. W. H. Edwards has since pointed out several beautiful cases of seasonal dimorphism in America. Thus
[Eng. ed. I learn by a written communication from Dr. Speyer that two Geometræ,