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iited certain functions to the ovum and certain others to the male cell does not, of course, prove that there is no difference in the functions of these elements; but in modern times we actually find that thinkers have gone to this opposite extremity of the subject, and have either tacitly implied or directly accepted the view that the two sexual elements play similar parts in heredity.
Neither Haeckel's hypothesis nor Jager's recognizes any difference in their functions, while Jager seems to believe, and Darwin explicitly states, that their shares in hereditary transmission are alike.
Many facts indicate that this view is, to say the least, very improbable, and I will give, briefly, a statement of some of the arguments against it, and will then devote a little space to a discussion of the reasons which have been given by Darwin and others for accepting it.
The structural difference between the ovum and the male cell is one of the most widespread and fundamental characteristics of organic beings, and it is found in all except the very lowest animals and plants. It is, to say the least, very improbable that a structural difference so fundamental and so nearly universal should have no functional significance, and the fact that in many marine animals, when the ripe unfertilized ova are thrown out into the ocean, like the male fluid, to be swept away by the tide, the sexual elements differ in the same way that they do in animals whose eggs are fertilized inside the body of the female, forbids us to believe that the difference depends simply upon the fact that the male cell must make its way to the ovum.
Many of the secondary characteristics of the ovum, such as its great size in birds and reptiles, and the presence in it of food-material in so many animals, are no doubt traceable to the fact that, in most animals, the egg is stationary, while the male cell can be conveyed from place to place; but we must believe that there is some more fundamental and primitive difference.
Even if the phenomena of Parthenogenesis did not show us that the part played by the ovum is more essential to the perpetuation of the race than the part played by the male cell, we should still be justified in the belief that the difference in form corresponds to some profound difference in function, and the possibility of Parthenogeuesis shows beyond question that this is the case.
Siebold has proposed the term parthenogenesis for the power which is possessed by certain female animals, especially the arthropods, to produce descendants without sexual union with a male.
The existence of this power was first pointed out by Aristotle (De Generatione Animalium, Lib. III., Cap. 10, 21, 22, 23). As this remarkable observer had no means for exact research at his command, he was, of course, unable to make use of rigid tests, or to furnish the severely exact proofs which have been given us by more modern naturalists; but he gives many reasons for suspecting that the unfertilized eggs of the honey-bee may give rise to perfect animals without sexual union; and although we now know that some of the reasons he urges do not really prove the case, yet modern science has given the most conclusive proofs of the correctness of his general conclusion.
I shall devote considerable space to this subject in order to show the unscientific reader that the existence of fertile virgin female animals is proved by the observations 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 docs 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, Schaffer, of Eegensburg, discovered its occurrence in fresh-water crustacea, although Dr. Albrecht had made the same discovery in insects in 1701. Schaffer 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 Schaffer'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.
Schaffer'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. Schaffer, the discoverer of parthenogenesis in Daphnia, also discovered that Apus, a crustacean which belongs to another order, lays eggs which give rise without impregnation 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 Bombycidae, 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, Bartbel