1 78 CAUSES OF VARIATION 



female and her germ cell are involved. But individuals thus 

 arising through unisexual reproduction vary ividely, a fact easily 

 credited when the phenomena of reduction are remembered. 



Parthenogenesis being limited to lower animals, the range and 

 character of variations are for the most part difficult of detection 

 and measurement. It is known, however, that great differences 

 in size occur among individuals parthenogenetically produced, 

 and characters generally are so variable in such individuals 

 as to lead to the statement that the variability of offspring 



never mates but once) she is, of course, capable of laying only unfertilized or " drone 

 eggs." In this way, in crossing, the drones and the workers may actually be of 

 different breeds. 



Plant lice reproduce parthenogenetically during the summer season, producing 

 only females ; but as the temperature lowers with approaching autumn a mixed 

 brood of both males and females appears, which, upon mating, produces the long- 

 lived, winter-enduring eggs. It is noteworthy that the parthenogenetic eggs of 

 bees develop males only, while those of plant lice develop females during the 

 summer, both sexes appearing as autumn approaches. 



It is also noteworthy that under the artificial heat of greenhouses, approximat- 

 ing perpetual summer conditions, parthenogenesis continues indefinitely, and males 

 are not produced unless the plants become badly dried up. 



Parthenogenesis differs greatly in degree. It is supposed to be complete in 

 certain minute crustaceans and in many rotifers among which " no males have ever 

 been found." It is "seasonal" in the aphis (plant lice), and "partial" in the 

 honeybee and " in some of the lower animals which are not themselves normally 

 parthenogenetic, but have relatives which are." Occasional parthenogenesis has 

 been frequently observed. An example is the silk moth, in which Nussbaum 

 found that out of 1 102 unfertilized eggs ... 22 developed ... up to a certain 

 point. It is supposed that in all cases of parthenogenesis many eggs fail to develop. 



In this connection it is noteworthy and extremely suggestive that among higher 

 animals frogs, hens, and even mammals the unfertilized ovum occasionally 

 begins segmentation, never proceeding far, however, on its parthenogenetic course. 



The student should understand that in all probability a large number of eggs 

 fail to develop into complete individuals even in the most successful parthen- 

 ogenesis, just as do many fertilized ova fail along the way (see Weismann, Essays 

 on Heredity, I, 175). There are all degrees of parthenogenesis, from the perfectly 

 successful down to zero. 



Bearing upon the general subject are the interesting experiments of Loeb in 

 artificial parthenogenesis, especially of the sea urchin, normally bisexual, but which, 

 after immersion in a saline solution of high density and subsequent return to nor- 

 mal sea water, commenced segmentation and afterward developed living larvae. 

 Magnesium and potassium salts proved most effective, though in general any 

 treatment avails that serves to withdraw a portion of the water from the unfertilized 

 egg (see Loeb in American Journal of Physiology, III, 434; also Loeb, Studies 

 in General Physiology, Part II, pp. 576-626, 638-692; Methods, pp. 766-772; 

 Geddes and Thomson, Evolution of Sex, pp. 183-198). 



