XIX.] MORPHOLOGY. 231 



ever, except those very low ones that have no distinction 

 of parts or of sexes. The fourth occurs only among com- 

 paratively low organisms. Among the higher organisms, 

 the different cases are perfectly distinct ; but among vege- 

 table and comparatively low animal organisms, they gra- 

 duate into each other : indeed, where individuality is very 

 indefinite, it may be said that all the four are particular 

 cases of homology between parts of the same organism. 



To make this last statement intelligible, it is necessary Homo- 

 to take an instance ; and the most obvious and familiar thf^l^'arts 



instance is that of a tree in flower. The tree at once bears of a tree in 



flower. 

 young buds and branches with fully developed leaves. 



These may be regarded either as different parts of the 



same organism, or as distinct organisms of the same 



species in different stages of development. The tree also, 



in most cases, bears flowers of both sexes ; and if the male 



elements are in one set of flowers and the female in 



another (which is a very common arrangement), the two 



sets of flowers may be regarded either as diflPerent organs 



belonging to the same organism, or as distinct organisms of 



the same species, but of opposite sexes. 



Metagenesis, in the usx;al 'sense of the word, seldom Meta- 



occurs among flowering plants, but it is common among Hy^ro^oa. 



the Hydrozoa,^ which, though u.nmistakeably animal, have 



a very remarkable resemblance to plants, both in the way 



in which single individuals or "morphological units" 



are united into compound ones, and in the relation of 



the generative organs to the whole compound organism. 



Among some Hydrozoa the generative organs consist of 



flower-like expansions, which never become detached from Flower- 



the body of the Hydrozoon which has produced them, and organs. 



which are precisely analogous to flowers. But in other cases 



these flower-like expansions acquire a mouth and tentacles 



so as to be fitted for living alone. They then break off, 



swim away, and in many cases grow many times larger 



than the stock they have left, before they mature the gene- 



1 It is, I think, to be regretted that the unmeaning word hydrozoa has 

 been substituted for the beautiful and most appropriate word zoophytes, 

 or animal plants. 



