14 EMBRYOLOGY. 



an individual which produces by sexual means gives origin to 

 asexual individuals differently organised 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) t\vo 

 different asexual generations may be interpolated between the 

 sexual generations. In all these cases the origin of the pheno- 

 menon 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 themselves 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 asexual ly. 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 alter- 

 nation of generations became established. The above is no 

 merely speculative history, since all gradations between com- 

 plete 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 

 Entoparasitic Trematodes and most Cestodes, is to be explained 

 in a slightly different way. 



It appears that in these parasitic forms a complicated meta- 

 morphosis 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. 



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. 



