210 LEISURE-TIME STUDIES. 



throughout the animal world regarded as necessary for the 

 production of eggs capable of developing into offspring. 

 Strangely enough, however, these wingless females not only 

 produce eggs, hatching them within their bodies, but the 

 eggs develop into beings exactly resembling themselves ; not 

 a single male aphis being represented within the limits of 

 this Amazonian population. Seven, eight, nine, or even 

 eleven generations of these wingless females may be pro- 

 duced in this manner, and the swarms of plant-lice which 

 infest our vegetation attest the fertility of the race. But in 

 the last brood of these insects, produced towards the close 

 of autumn, winged males appear in addition to the females, 

 which latter also possess wings. The members of this last 

 brood produce eggs of ordinary nature, which lie dormant 

 during the winter, but which in the succeeding spring will 

 inaugurate the same strange life-history through which their 

 progenitors passed. The case of the plant-lice may for the 

 present be dismissed with the observation that the law of 

 heredity appears to operate in this instance in a somewhat 

 abnormal, or at any rate in a very unusual, manner. The 

 true similitude of the winged parents is not attained until 

 after the lapse of months, and through the interference, 

 as it were, of many generations of dissimilar individuals. 

 No less worthy of remark is the circumstance that one sex 

 alone is capable of giving origin to new beings, which sooner 

 or later produce in turn the natural duality of sex, forming 

 the rule of both animal and plant creation. And the case 

 of the plant-lice is rendered the more remarkable by the 

 consideration that of 58.000 eggs laid by female silk-moths 

 which were separated from the opposite sex, only twenty- 

 nine developed into perfect caterpillars, the female plant- 

 lice possessing a fertility under like circumstances which 

 would be amazing even if taking place under the normal 

 laws and conditions of development. 



Cases of the unusual development of animals, which 

 serve as parallel instances to the case of the plant-lice, are 

 by no means rare. Thus in the case of the star-fishes, sea- 



