INTRODUCTION. 11 



The simplest cases are those in which an individual which produces 

 by sexual means gives origin to asexual individuals differently or- 

 ganised to itself, which produce by budding the original sexual form, 

 and so complete a cycle. Instances of this kind are supplied by the 

 Hydrozoa, Annelida and Tunicata. In the case of the Tunicata 

 (Doliolum) two different asexual generations may be interpolated 

 between the sexual generations. In all these cases the origin of the 

 phenomenon is easily understood. It appears, as is most clearly shewn 

 in the case of the Annelida, that the ancestors of the species which 

 now exhibit alternations of generations originally reproduced them- 

 selves at the same time both sexually and by budding, though 

 probably the two modes of reproduction did not take place at the 

 same season. Gradually a differentiation became established, by 

 which sexual reproduction was confined to certain individuals, which 

 in most instances did not also reproduce asexually. After the two 

 modes of reproduction became confined to separate individuals, the 

 dissimilarity in habits of life necessitated by their diverse functions 

 caused a difference in their organization ; and thus a complete 

 alternation of generations became established. The above is no 

 merely speculative history, since all gradations between complete 

 alternations of generations and simple budding combined with sexual 

 reproduction can be traced in actually existing forms. 



The alternation of generations as it is found amongst the Ento- 

 parasitic Trematodes and most Cestodes, is to be explained in a 

 slightly different way. 



It appears that in these parasitic forms a complicated metamor- 

 phosis first arose from the parasite having to accommodate itself 

 to the different hosts it was compelled to inhabit, owing to the 

 liability of its primitive and subsequent hosts to be devoured 1 . A 

 capacity for asexual multiplication obviously of immense advantage 

 to a parasite appears to have been acquired in some of the 

 stages of this metamorphosis, and an alternation of generations 

 thus established. 



A nearly parallel series to that exhibiting alternations of sexual 

 generations with generations which produce by budding is supplied 

 by the cases where sexual generations alternate with parthenogenetic 

 ones, or in some instances even with larvae which reproduce sexually 

 or else parthenogenetically. 



The best known examples of this form of alternations of genera- 

 tions are found amongst the Insecta 2 . A simple case is that of the 

 Aphides. The ova deposited by impregnated females give rise 

 to forms differently organised to the parents but provided with an 



1 The appearance of Vertebrata on the globe as the forms which most frequently 

 preyed on Invertebrate forms, and were themselves not so liable to be devoured, has no 

 doubt had a great influence on the metamorphosis of internal parasites, and has 

 amongst other things resulted in these parasites usually reaching their sexual state in 

 a vertebrate host. 



2 For details vide Chapter on Insecta. 



