NEMA TO CERA. 43 



The explanation of this alternation of asexual and sexual genera- 

 tions is the same as that given to the phenomena observable in the 

 Aphides. 



This parthenogenetic development has a very limited range in the 

 animal kingdom, being found only in Arthropods and Rotifers. It 

 has no doubt been derived in a secondary way from sexual repro- 

 duction. Some most remarkable cases are found in insects such as 

 bees and saw-flies, in which J 's are produced without any fertiliza- 

 tion, contrary to the usual rule, where ? 's are produced when no 

 fertilization has taken place, and these give rise to further generations 

 of females. Eventually through a change of climatic conditions and 

 food both males and females are produced. 



The existence of sexual with asexual parthenogenetic reproduction 

 has led to that remarkable phenomenon in the animal kingdom 

 called "alternation of generations," so typically displayed in the 

 Coelenterates and Annelids. These furnish us with the most simple 

 forms of " alternation of generations," viz., a sexual form giving rise 

 to an asexual form which, by a process of budding, gives rise to 

 sexual forms like the one from which it started. This form of 

 alternation is easily understood. The ancestors of the species 

 originally produced themselves sexually and by budding at the same 

 time, but most likely at different seasons of the year. By degrees 

 the two modes of reproduction became confined to separate indi- 

 viduals, and then gradually the dissimilarity in habits caused their 

 structure to become considerably modified ; and thus gradually a 

 complete alternation of generations became-established. 



A closely-allied mode to the one above (sexual and budding) is 

 seen in the case of those animals where sexual reproduction alter- 

 nates with parthenogenetic reproduction, or with larvae that may 

 produce sexually or parthenogenetically. The Aphides may be 

 taken as an example of this. In this case the ova develop partheno- 

 genetically in the oviduct of the female, and as long as warm weather 

 and plenty of food remain this mode of reproduction continues, but 

 as soon as these conditions fail, sexual ( $ and ? ) individuals appear. 

 We may assume here the capacity of the females for producing 

 parthenogenetic young without the intervention of the male element 

 has been found advantageous, and has been taken hold of and 

 moulded by natural selection and has led to the permanent pro- 

 duction of viviparous parthenogenetic young, which only change to 

 male and female forms at the cessation of favourable conditions. 

 The same explanation applies to the Cecidomyia larvas as mentioned 

 above. 



