OTHER FORMS OF ALTERNATION. 189 



of forms is as generally recognizable at an advanced stage of 

 the life of the higher organisms, in the phenomena atten- 

 dant on the maturation of the reproductive organs, as we 

 have seen it to be in the initial period of tlieir embryonic 

 development. 



The rationale of the difference is this, that in the higher 

 species the prevailing law of centralization prevents the 

 structures developed for the performance of the sexual func- 

 tion, from ever acquiring the automatic organization and in- 

 dependent position that would entitle them to be recognised 

 a« distinct zooids. Such a result is confined to the lower 

 forms of life, and indeed occurs but exceptionally even 

 among them. Even in tribes, where alternation is the rule, 

 there occur species in which this process of sexual matura- 

 tion is so curtailed, by the suppression of structures com- 

 monly present in their congeners, that instead of separate 

 zooids we have merely organs of reproduction of the simplest 

 possible construction. 



But the instances which have been given of transition 

 forms, concur with the general considerations just adverted 

 to, in suggesting the conclusion of an essential community 

 of nature in the processes connected with the development 

 of the structures ministering to this function, whether they 

 put on the exceptional character of distinct zooids, or ap- 

 pear in the more usual form of constituent organs, in the 

 unity of the body which produces them. 



