This summary of the more interesting phenomena of variation presented by the eastern Papilionidæ is, I think, sufficient to substantiate my position, that the Lepidoptera are a group that offer especial facilities for such inquiries; and it will also show that they have undergone an amount of special adaptive modification rarely equalled among the more highly organized animals. And, among the Lepidoptera, the great and pre-eminently tropical families of Papilionidæ and Danaidæ seem to be those in which complicated adaptations to the surrounding organic and inorganic universe have been most completely developed, offering in this respect a striking analogy to the equally extraordinary, though totally different, adaptations which present themselves in the _Orchideæ_, the only family of plants in which mimicry of other organisms appears to play any important part, and the only one in which striking cases of polymorphism occur; for such we must consider to be the male, female, and hermaphrodite forms of _Catasetum tridentatum_, which differ so greatly in form and structure that they were long considered to belong to three distinct genera. _Arrangement and Geographical Distribution of the Malayan_ Papilionidæ. Although the species of Papilionidæ inhabiting the Malayan region are very numerous, they all belong to three out of the nine genera into which the family is divided. One of the remaining genera (_Eurycus_) is restricted to Australia, and another (_Teinopalpus_) to the Himalayan Mountains, while no less than four (_Parnassius_, _Doritis_, _Thais_, and _Sericinus_) are confined to Southern Europe and to the mountain-ranges of the Palæarctic region. The genera _Ornithoptera_ and _Leptocircus_ are highly characteristic of Malayan entomology, but are uniform in character and of small extent. The genus _Papilio_, on the other hand, presents a great variety of forms, and is so richly represented in the Malay islands, that more than one-fourth of all the known species are found there. It becomes necessary, therefore, to divide this genus into natural groups before we can successfully study its geographical distribution. Owing principally to Dr. Horsfield’s observations in Java, we are acquainted with a considerable number of the larvæ of Papilios; and these furnish good characters for the primary division of the genus into natural groups. The manner in which the hinder wings are plaited or folded back at the abdominal margin, the size of the anal valves, the structure of the antennæ, and the form of the wings are also of much service, as well as the character of the flight and the style of coloration. Using these characters, I divide the Malayan Papilios into four sections, and seventeen groups, as follows:— Genus ORNITHOPTERA. a. _Priamus_-group. Black and green. b. _Pompeus_-group. Black and yellow. c. _Brookeanus_-group. Genus PAPILIO. A. Larvæ short, thick, with numerous fleshy tubercles; purplish.