48o LIFE : OUTLINES OF GENERAL BIOLOGY 



develop from unfertilised eggs; and the prolific summer generations 

 of greenflies or Aphides are parthenogenetic, no males occurring for 

 months, l^arthenogenesis is of frequent occurrence (i) in many of 

 the lower crustaceans, such as the brine-shrimp Artemia, the large 

 freshwater Apus, and some small "water-fleas", e.g. Daphnia, 

 Moina. Cypris, and Candona; (2) in some insects, notably among the 

 gall-wasps (Cynipidre), in certain species of which males have never 

 been found, and among saw-flies (Tenthredinidae) ; and (3) in most 

 of the minute Rotifers or "Wheel Animalcules." In most rotifers, 

 parthenogenesis is the rule; in some species males have never been 

 found; in some forms in which they occur they do not fertilise the 

 eggs. In most of the cases of parthenogenesis among crustaceans and 

 insects, males are absent for months or years, but reappear at 

 intervals. Among plants there are few examples of uninterrupted 

 or complete parthenogenesis in the strict sense, for it is necessary to 

 exclude relapses into asexuality, as seen for instance in many of the 

 lower Fungi, where the sexual reproduction has more or less degene- 

 rated. The development of an egg-cell without fertilisation is seen 

 in Chara crinita, one of the water-stoneworts, which is represented 

 in Northern Europe by female plants only. Parthenogenesis has 

 come to be the rule in the common dandelion, and it also occurs 

 in some hawkweeds and in a few other types, e.g. species of 

 Alchemilla and Antennaria. It may be noted here that there is no 

 reason whatever to associate the dominance of parthenogenesis 

 with any loss of racial vigour. A hundred successive parthenogenetic 

 generations have been carefully observed in the case of Daphnia, 

 and there was no suggestion of any degeneration. In a few cases the 

 occurrence of variation in forms produced parthenogenetically has 

 been demonstrated. 



It may be useful to distinguish several different grades of parthe- 

 nogenesis, {a) What may be called pathological parthenogenesis is 

 illustrated when the egg-cell, say of a hen, exhibits without fertilisa- 

 tion a number of divisions. In none of these cases has the develop- 

 ment been known to go far. {b) The term casual parthenogenesis 

 may be applied to cases where the occurrence is observed as a rare 

 exception, e.g. in silk-moths. It occasionally happens that worker- 

 ants, not normally reproductive at all, produce ova which develop 

 parthenogenetically. Since the discovery of what is called "artificial 

 parthenogenesis" (.see below), these instances of pathological and 

 occasional parthenogenesis have become more intelligible, (c) Partial 

 parthenogenesis is well illustrated by hive-bees. The queen receives 

 from the drone a store of male-elements or spermatozoa, and it rests 

 with her, in laying the eggs, to fertilise them or not. Those eggs 

 that are fertilised from the store of sj)ermatozoa in her spermothecae 

 develop into workers or queens (according to the nurture); those 

 that are not fertilised develop into drones. The same is true of some 



