T7 2 THE EVOLUTION OF SEX. 



the normal balance which makes division possible without the usual' 

 stimulus of fertilization. Abundant and at the same time stimulating 

 nutrition (Rolph), early differentiation of the sex-cells (Simon), the 

 general preponderance of reproductive over vegetative constitution 

 (Hensen), their liberation before the anabolic bias has carried them 

 too far, are among these favoring conditions. The incipient segmen- 

 tation observed in a few ova is an independent effort to save them- 

 selves from being too big to live, since they are not passive enough 

 to remain dormant. Waste has set in, self-digestion begins, the cell 

 is forced into the expedient of division. In higher animals this is 

 all in vain; in lower animals 'such imperfectly differentiated female 

 cells are commoner; they form the parthenogenetic ova. 



IX. Origin of Parthenogenesis. — From the occurrence of 

 parthenogenesis in the animal series, it is certain that it has originated 

 as a degeneration from the ordinary sexual process. It is no direct 

 persistence of a primitive ideal state, though to some degree a recapit- 

 ulation of it. One hypothetical mode of origin, which may well apply 

 to the rotifers, is easily sketched. In conditions favoring katabolism 

 the males wore themselves out, the females became katabolic enough 

 to do without them. We find the males, where they persist, much 

 smaller than the female rotifers, often extremely degenerate, in one 

 section wholly unknown. Again, from the fact that the interruption 

 of a parthenogenetic series of females by the appearance of males 

 usually occurs in hard times, we may infer the prosperous vital con- 

 ditions induced parthenogenesis. Why, then, are not internal para- 

 sites parthenogenetic? They are very generally hermaphrodites, and 

 have moreover gone beyond parthenogenesis to prolific asexual 

 multiplication. 



It is misleading to interpret the occurrence of parthenogenesis as 

 due to "motives" and "important advantages." These are after- 

 thoughts of our importations. It is not easy indeed to keep from 

 metaphorical language which suggests that polar globule-formation is 

 a "contrivance," and parthenogenesis a "device." Such casual 

 words are of little account; but to say, as Weismann does, "that 

 sexual reproduction has here been given up, not by any chance nor 

 from internal conditions, but from quite definite external grounds of 

 utility (zweckmassigkeitsgrunden)," is to say the least misleading. A 

 species of crustacean is being decimated by enemies, increased multi- 

 plication would lessen the danger of extinction, parthenogenesis is 

 established, and for every one before producing eggs there are now 

 i two — voila toftt. Against this short and easy method with Nature 

 we emphatically protest, and maintain that the origin of partheno- 



