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species can be reproduced simply by budding, but the result would, 

 if maintained for a number of generations, in the end prove disas- 

 trous to its integrity. Nature abhors self-fertilization. So that 

 while, as in these Hydroids, the zooid form may be produced by 

 budding, yet the time comes when the individuals of one colony 

 must mingle their reproductive elements with those of a remote 

 colony, through the medium of the water. By this mode of repro- 



or gemmation has for its object the extension of the colony of 

 nutritive and reproductive zooids. This alternation of budding 

 with sexual generation or "alternation of generations," or "parthe- 

 nogenesis," is first met with in the Hydroids, and we shall find it 

 often recurring in the higher animals when needed to meet some 

 special exigency of the species. 



