vations of a great number of competent naturalists; that the subject has been thoroughly and carefully studied, with every precaution against error, and that our belief in its existence does not rest upon the unverified statements of a few observers.

In this summary I shall give many references to authorities, but as my purpose is not to give a complete bibliography, but simply to show how thoroughly the subject has been studied, many names are omitted.

Most of the following facts are taken from Gerstecker's history of the subject in Volume v. of Bronn's Klassen und Ordnungen des Theirreichs, although I have referred to many of the original papers and have added many facts which are not mentioned by Gerstecker. The subject is perfectly familiar to most naturalists, and the amount of space devoted to it may seem unnecessarily great to such persons, but it is important to impress upon unscientific readers a sense of the exact and definite character of the evidence for the existence of parthenogenesis, and a short history of the subject seems the most effective means for accomplishing this purpose.

Among the crustacea and insects, parthenogenesis is by no means unusual. It occurs in some groups where impregnation by males is so nearly universal that naturalists have been slow to credit any exceptions. In other groups it is the general rule, and fertilization by a male is the exception. In some genera and species the power is shown only by a few individuals, while in others it is shared by all the females. In some cases the unfertilized eggs give rise to females only, in other cases to males, and in still other cases to both sexes.

In 1775, Schäffer, of Regensburg, discovered its occurrence in fresh-water crustacea, although Dr. Albrecht

had made the same discovery in insects in 1701. Schäffer found ("Abhandlungen von Insecten ") that when a female specimen of the common water-flea or Daphnia, a small fresh-water crustacean, is placed by itself immediately after it is born, and is kept throughout its whole life without any chance of union with a male, it gives birth to great numbers of young females, and that the isolation of these young specimens has no more effect upon their fertility than it had in the case of their mother, but that they continue to reproduce for an indefinite number of generations when all chance of access to a male is excluded.

This observation may be repeated by any one with the greatest ease, for Daphnia is very common in most fresh water ponds and streams, and it multiplies in confinement with great rapidity, so that there is no difficulty in verifying Schäffer's experiments, or in showing the correctness of his conclusions.

Certain authors have held that the parthenogenetic eggs of Daphnia are not true eggs at all, but simply internal buds (Lubbock, Phil. Trans., 147, p. 88), and that the so-called "winter eggs," which seem, in most cases at least, to require impregnation, are the true ova; but Weissmann, who has made a very thorough study of the origin of the ova in the ovary of Leptodora ("Ueber die Bildung von Wintereiern bei Leptodora hyalina," Zeit. f. Wiss. Zool., XXXV.), has shown that while there are some minor differences in the mode of origin of the two kinds of eggs, both are real ova in the strictest sense, and cannot be compared with buds.

Schäffer's experiments were independently repeated in 1820 by Jurine, and this observer not only reached the same result, but also proved that fertile winter eggs

may be produced by isolated females whose mothers and grandmothers had been isolated all their lives.

Claus has shown that the eggs begin to develop in the female Evadne, a form closely related to Daphnia, before the animal is born; and impregnation would here seem to be impossible.

In Daphnia and related forms the parthenogenetic eggs appear to give rise to females only, but as the males are very rare indeed, as compared with the females, it is difficult to show that they never originate by parthenogenesis, for the evidence is only negative. Schäffer, the discoverer of parthenogenesis in Daphnia, also discovered that Apus, a crustacean which belongs to another order, lays eggs which give rise without impreg'nation to fertile females, and that this may go on for an indefinite number of generations. In Apus, and in most of its allies, the males are extremely rare, although the females may be very abundant, and one observer, Joly, found only one male specimen of Artemia salina among 3000 females.

Parthenogenesis is known to occur in many insects. It is rare and exceptional in some of them, while in others it is as frequent and normal as it is in Daphnia.

Among the butterflies and moths, sexual union is the rule, and parthenogenesis a rare exception, but in 1701 Dr. Albrecht made the remarkable discovery that a female Bombyx, which had escaped from its pupa under a glass shade, and which could not have been visited by a male, laid fertile eggs. As sexual union is known to be almost universal in the Bombycidæ, this observation was at first discredited, but the phenomenon has in more modern times been observed with every possible precaution in Bombyx mori by a number of most competent observers, among whom are Schmidt, Barthél

emy, Jourdan, Siebold and others. They all agree that while parthenogenesis is rare in this species, it does sometimes occur, and it is known that the parthenogenetic eggs give rise to fertile males and fertile females, which may unite sexually and thus produce fertile eggs. Dr. Kipp has reared another form, Smerinthus populi, from eggs fertilized by a male which hatched from a parthenogenetic egg, and laid by a female which had been reared in the same way.

In Bronn's Klassen und Ordnungen, Gaerstecker gives the following list of moths in which parthenogenesis has been observed, with the name of the observer. The list might be greatly enlarged by the addition of cases which have been recorded since its compilation, but it is sufficient for our purpose, which is simply to show that the fact has been verified repeatedly by many ob

servers.

Sphinx ligustri, once.....

Treviranus.

Smerinthus populi, four times....Nordmann, Brown, Newnham,

Smerinthus ocellatus, once..

Kipp.

.Johnston.

Euprepia caja, five times.... Brown, Lehocq, Robinson, Schlapp,

Although these cases make a long list, which might be greatly increased, they are still exceptional, for in all these species almost all the eggs fail to develop unless they are fertilized by a male; but in some other groups of insects parthenogenesis occurs more frequently, and seems to be perfectly normal. The most remarkable instances are those which occur in the social insects, such as the bees.

It is well known that a community of honey-bees consists of individuals of three kinds-the workers or rudimentary females, which are the most numerous; the perfect females or queens, of which only one is usually present in a hive; and the drones or males.

In the workers, or as they are sometimes falsely called the neuter bees, the female reproductive organs are very imperfectly developed: the vagina is so small that union with a male is hardly possible, and the receptaculumseminis is very rudimentary, yet it is well known to all bee-cultivators that they do sometimes lay eggs which are capable of development, not only in the honey-bee but in other species also. Among the honey-bees such fertile workers are always found in a hive which has lost its queen, and they have been called "drone mothers," from the fact that their eggs produce only drones or males.

The queen-bee is the only member of the hive which unites sexually with the males, and her reproductive organs are very large and well developed, as contrasted with those of the worker. Her receptaculum-seminis is large enough to retain a sufficient supply of the male fluid to serve for fertilizing great numbers of eggs, and it is usually found to contain a considerable quantity. Sexual union takes place during flight, and queens with imperfect wings are never impregnated, and Siebold,