292 Causes and Course of Organic Evolution 



wavering between asexual swarmspore formation of new 

 plants; formation of these by parthenogenesis, but in which 

 the swarmspores show a tendency first to weak sexual differ- 

 entiation; and formation of these by true sex fusion of like 

 or unlike gamete cells. Studies of the past quarter century 

 have equally shown that, amongst the higher plants and ani- 

 mals, wavering transitions from natural or artificial partheno- 

 genesis to union of sex products may occur. Thus in Char a 

 and Marsilea amongst cryptogams; in Thalictrum, Alchemilla, 

 and Antennaria amongst flowering plants; in many rotifers, 

 smaller crustaceans, and genera or even groups of insects, 

 frequent and extensive parthenogenesis occurs alongside occa- 

 sional sex fusions. Loeb's experiments also wiih. echinoderms 

 prove that artificial parthenogenesis may be ensured by treat- 

 ing the eggs with various kinds and strengths of solution. 



The question then arises: what is the morphological and 

 physiological state of the egg, that causes it to return from the 

 sexually mature to the more primitive parthenogenetic con- 

 dition.'* This seems to be due amongst animals and probably 

 also amongst plants to the relative amount of chromatin re- 

 tained in or thrown out from an egg as one or more polar bodies. 

 In eggs that are normally fertilized, two small chromatin masses 

 or polar bodies are throTvn out of the egg in succession, one 

 of which removes half of the chromosomes, and so expels half 

 of the cognitic or main hereditaiy substance of the egg. In 

 parthenogenetic eggs this half is retained, only one polar body 

 being extruded. 



Second: If such be true some evolutionary cause must have 

 existed that gave rise to the parthenogenetic state with its 

 one polar body, in place of the usual prefertilization state 

 with its two extruded polar bodies; and in place of the most 

 primitive condition of all, as seen in simpler algae, where asexual 

 cells alone exist. Amongst the five factors of organic evolu- 

 tion already cited (p. 204) the only two which seem to be here 

 concerned — either one alone or both combined — are envir- 

 onment and reproduction. Now, in the rei)roduction of such 

 colloid masses as Traube's cells, the two factors wliich might 



