XXll INTRODUCTION. 



It often happens that the zooids resulting from 

 ■fission or gemmation are very dissimilar in out- 

 ward appearance to the organisms by which they 

 were produced, while they possess the power of 

 giving rise, by a true generative process, to be- 

 ings exactly resembling the latter. In order 

 to explain these phenomena, certain naturalists, 

 ignorant of the distinction between "zooid de- 

 velopment" and sexual generation, devised the 

 ingenious theory of " alternation of generations." 

 But, from what has been said, it must appear 

 that, in the instances referred to, there is not an 

 alternation of two (or more) distinct generative 

 acts, but rather an alternation of true generation 

 with either gemmation or fission. 



The functions of Eolation may be di\aded into 

 those of the muscular apparatus and those of the 

 nervous system. 



Were it possible for us to become acquainted 

 with the structme and functions of any organism, 

 our knowledge of its natm-e would still be im- 

 perfect so long as we remained ignorant of its 

 life history. Hence the necessity of the study of 

 Development, which investigates the primitive 

 characters of living beings, and the changes which 

 they undergo in passing from the embryonic to 

 the adult condition. Morphology, it may be said, 

 teaches us what an animal ^s, Physiology what it 



