History of the Theory of Heredity.. 57 



had made the same discovery in insects in 1701. Schiif- 

 fer found ('VAbhandlungen von Insecten") that when 

 a female specimen of the common water-flea or Daplmia, 

 a small fresh- water crustacean, is placed by itself im- 

 mediately after it is born, and is kept throiighuut its 

 whole life without any chance for union with a male, it 

 gives birth to great numbers of young females, and that 

 the isolation of these young specimens has no more ef- 

 fect upon their fertility than it had in the case of their 

 mother, but that they continue to reproduce for an in- 

 definite number of generations when all chance of access 

 to a male is excluded. 



Tills 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 con- 

 finement with great rapidity, so that there is no diffi- 

 culty 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 egns at all, but simply 

 internal buds (Lubbock, Pliil. 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 Weismann, who has made a very thorough 

 study of the origin of the ova in the ovary of Leptodora 

 ('^Ueber die Bildung von Wintcreiern 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 w^ere independently repeated 

 in 1820 by Jurine, and this observer not only reached 

 the same result, but also proved that fertile winter eggs 



