136 THE NATURE AND VARIETIES OF 



7. The conclusion, therefore, at which we arrive is 

 this, that each of the three stages into which the life history 

 has been divided (protomorphic, orthomorphic, and gamo- 

 morphic), may become the scene of a process of gemmation, 

 attended by an alternation of forms : and, under one or 

 other of the varieties of alternation thus admitted, I am 

 persuaded all the cases known, in both kingdoms of nature, 

 may readily be ranged. 



To the protomorphic, I would refer, along with the 

 Trernatoda, the Echinodermata among animals, and the 

 Mosses and Hepaticse among plants ; to the orthomorphic, 

 I would refer such Crustacea as Daplmia and Cyclops, 

 whose reproduction is, to some extent, parallel with that of 

 the Aphides* ; and to the gamomorphic, the Salpce in the 

 animal, and the Ferns and their allies in the vegetable 

 kingdom. 



It is not common, as has been stated, to meet with the 

 concurrence of more than one kind of alternation in the 

 same species. The Cestoid worms, however, have been 

 already mentioned, as furnishing examples of alternation, 



of the Trematoda and the Aphides. Very probably, indeed, notwith- 

 standing the distinction now indicated, the two modifications may be con- 

 nected by a series of intermediate gradations. All such precursory forms, 

 being sexless, come under the term " agamozooid,' used by Huxley and 

 Lubbock. See Carpenter's Compar. Physiology, 4th Ed., p. 558, 597, 

 529. 



* The species of the allied family of Coccida, and of the transitional 

 genus Chermes, present, according to Leuckart, phenomena somewhat 

 analogous to those occurring in the reproduction of the Aphides (Annals 

 of Nat. Hist., 3d Ser., IV., 321). Among the cases of orthomorphic al- 

 ternation should probably be included also that of the Pteromalus, as de- 

 scribed by Filippi. Here a caterpillar is developed within a maggot-like 

 larva, which by its growth it reduces to a mere sac, and in due time 

 passes into the pupa state, and comes forth as an insect of the Ichneumon 

 tribe. The case may possibly be one of parasitism, but the uniformity of 

 the occurrence is opposed to this view. See Annals of Nat. Hist., 2d 

 Ser., Vol. IX., 461, and Carpenter's Princip. of Compar. Phys., 4th Ed., 

 597. 



