XII.] CONJUGATION AND SEXUAL REPRODUCTION 1 59 



the germ-nucleus ^ of the ^%% produced by the ' reducing 

 division,'— 10 rt ; 9<7 + iZ>;8^ + 2Z>; 7« + 3Z>;6« + 4Z>;5« + 5^; 

 4<^ + 66; 3« + 7<^; 2rt + 8Z>; 1^ + 9^; ioZ>. 



Hence we see that out of eleven possible combinations there 

 are only two which contain one kind of idant alone : all others 

 contain both. In the case of twenty idants there are only two 

 out of forty-one combinations which contain either a ov b alone ; 

 with forty idants, only two out of eighty-one. 



Naturally this does not imply that the diminution to one kind 

 of idant is improbable, but only that it would always remain 

 largely in the minority, i.e. it would be found in relatively very 

 few cases among the numerous eggs of the same mother. 

 This must, however, change in the course of generations ; for 

 only in one out of the eleven combinations are a and b present in 

 equal numbers, and only in the descendants of this single 

 variety will the germ-plasm be chiefly made up o^a and b in equal 

 proportions : in all the other ten combinations, either a ov b 

 preponderates, and according to the extent of preponderance is 

 the probability of a greater or less number of eggs which contain 

 only a or only b. We may therefore maintain that, by continued 

 parthenogenesis, the germ-plasm becomes ever simpler as 

 regards its composition oat of ids until it comes to consist of 

 only two kinds of idants, but when once this composition has 

 been reached it may be retained through long periods of time, 

 during which there will be a changing majority, sometimes 

 of one and sometimes of the other kind. Among the eggs of 

 such a female there would always be some in which the germ- 

 plasm would contain only one kind of idant. 



Observations on Inheritance in Parthenogenesis. 



When I developed the idea that the essential meaning of 

 sexual reproduction was to ensure that amount of individual 

 variability which is necessary for the phyletic development of 

 the organic world by means of natural selection, I inferred that 

 uninterrupted parthenogenetic reproduction would prevent the 



^ I have employed Strasburger's term ' germ-nucleus ' instead of 

 '■ segmentation nucleus ' which has been commonly used up to this time, 

 as a general term for the nucleus of the mature egg from which embryonic 

 development proceeds, whether parthenogenetic or amphigonic. 



