In those kinds of butterflies that attain the perfect state twice in the year, the individuals composing the first flight are somewhat different in marking from those of the second flight. Such species as the large and small whites exhibit this kind of variation, which is termed seasonal dimorphism. The males of some species, as for example the Common Blue and the Orange-tip, differ from the females in colour; this is known as sexual dimorphism. The Silver-Washed Fritillary, which has two forms of the female, one brown like the male, the other green or greenish in colour, is a good example of dimorphism confined to one sex. Gynandrous specimens, sometimes called "Hermaphrodites," are those which exhibit both male and female coloration, or other wing characters; when one side is entirely male and the other side entirely female, the gynandromorphism would be described as complete. The ornamentation on the under side of a butterfly differs from that of the upper side, and is found to assimilate or harmonize in a remarkable manner with the usual resting-place. It is therefore of service to the insect when settled with wings erect over the back, in the manner of all butterflies, except some few kinds of Skippers. The number of known species of butterflies throughout the world has been put at about thirteen thousand, and it has been suggested by Dr. Sharp that there may be nearly twice as many still awaiting discovery. Dr. Staudinger in his "Catalog" gives a list of over seven hundred kinds of butterflies as occurring in the whole of the Palæarctic Region. This zoological region embraces Europe, including the British Islands, Africa north of the Atlas range of mountains, and temperate Asia, including Japan. The entire number of species that can by any means be regarded as British does not exceed sixty-eight. Even this limited total comprises sundry migratory butterflies, such as the Clouded Yellows, the Painted Lady, the Red Admiral, the Camberwell Beauty, and the Milkweed Butterfly; and also the still less frequent, or perhaps more accidental visitors, the Long-tailed Blue and the Bath White. Again, the Large Copper is now extinct in England, and the Mazarine Blue does not seem to have been observed in any of its old haunts in the country for over forty years. The Black-veined White is also scarce and exceedingly local. The majority of the remaining fifty-seven butterflies may be considered natives, and of these about half are so widely distributed that the young collector should, if fairly energetic, secure nearly all of them during his first campaign. The other species will have to be looked for in their special localities, but a few kinds are so strictly attached to particular spots, that a good deal of patience will have to be exercised before a chance may occur of obtaining them. A few remarks may here be made in reference to the names and arrangement adopted in the present volume.