I74 THE EVOLUTION OF SEX. 



SUMMARY. 



I. Parthenogenesis was formerly believed to be of wider occurrence than 

 it really is, but it is definitely known to be not uncommon in lower animals. 



II. Artificial, pathological, occasional, partial, seasonal, juvenile, and total 

 parthenogenesis must for clearness be distinguished. 



III. The occurrence of parthenogenesis is especially well seen in rotifers, 

 crustaceans, and insects. 



IV. It is rare among plants, but certainly occurs in some of the lower 

 forms. 



V. The offspring of parthenogenetic ova is very diverse. 



VI. The effects of parthenogenesis on the species deserve consideration, 

 especially by those who find in sexual intermingling the sole fountain of specific 

 variation. 



VII. Parthenogenetic ova, so far as observed, form only one polar body. 



VIII. Parthenogenetic ova are here regarded as imperfectly differentiated 

 female cells, retaining certain male or katabolic characteristics. 



IX. In origin parthenogenesis is regarded as a degeneration from the 

 ordinary- sexual process. 



X. The voluntary parthenogenesis of bees is taken as a concrete illustration. 



LITERATURE. 



See especially the already cited works of Balfour, Brooks, Hensen, Minot, 

 Rolph, Sachs, Weismann; also — 



Owen.— Parthenogenesis ; or, The successive production of Procreating Indi- 

 viduals from a single ovum. London, 1S49. 

 Von Siebold. — Beitriige zur Parthenogenesis. Leipzig, 1S71. 

 Leuckart.— Art "Zeugung" in Wagners's Handworterbuch d Physiol., Bd. 



IV., 1853. 

 Gerst.ecker. — Bronn's Klassen und Ordnungen des Thierreich, Vol. V. 



Arthropoda. 

 Brooks, W. K. — Law of Heredity. Baltimore, 1883. 

 Simon, F.— Die Sexualitat, &c, Inaug. Dissertation. Breslau, 18S3. 

 Blochmann.— Uber die Richtungskorper bei Insekteneiern, Biolog. Cen- 



tralblatt, VII., and Morpholog. Jahrbuch, XII. 

 Weismann, A. — Beiter, zur Naturgeschichte der Daphnoiden. Leipzig, 1876- 



79. Uber die Zahl der Richtungskorper und iiber ihre Bedeutung fur die 



Vererbung. Jena, 1887. 

 Weismann, A., and Ischikawa, C. — Brichten der nautrforsch. Gesellschft., 



Freiburg, III., 1887. 

 Hudson and Gosse. — The Rotifera. London, 18S6. 

 Plate. — Beitrage zur Naturgeschichte der Rotatorien, Jenaische Zeitschft. f. 



Naturwiss, XIX., 1886. 

 Karsten, H. — Parthenogenesis und Generations-Weschel im Thier und 



Pflanzenreiche. Berlin, 1888. 



