Malaco- 

 straca, 



272 HABIT AND INTELLIGENCE. [chap. 



from the egg. Of course it is not probable that the de- 

 velopment of the Nanplius into any malacostracan form 

 was effected in less than thousands, perhaps millions, of 

 generations. 



Other 3. ]\Ialacostracans pursuing the same course of deve- 



lopment as the Penoeus, except that they leave out the 

 nauplius stage and begin as zoeas. 

 Fresh- 4. Fresh-water malacostracans developing by a direct 



Mafaco- pi'ocess from the egg into the mature form. 

 straca xhe fact of the development of the fresh- water species 



their meta- beiug directj is very significant. The fresh- waters have, 

 morphoses j^. jg tolerably certain, been colonized from the sea, and 



by vana- "^ 



tion. not the reverse : the change from salt to fresh water, like 



any other change, must have acted as a stimulus to varia- 

 tion ; and variation, among other modes of action, must 

 have, in some cases at least, the effect of causing the 

 young to acquire the characters of the parent at birth 

 instead of by a subsequent metamorphosis. 

 Seiies iu I have kept the case of the Hydrozoa for the last. In 

 Hy lozoa. ^^ ^^ havc to spcak, not of metamorphosis, but of meta- 

 genesis. In that class we find the following series : — 

 Hytlra. 1. In the common fresh-water Hydra, the generative 



products are matured in organs which are mere swellings 

 on the surface of the body. 

 Hydrozoa 2. In other members of the class the generative organs 

 I"* ,., are distinct flower-like exj)ansions. From the mere 

 generative swelliugs of the Hydra to the flower-like organs of the 

 organs. campanularian and sertularian Hydrozoa, there is a 

 regular gradation ; and the same anatomical elements are 

 discernible through all the various forms of that gradation, 

 much in the same way that the same anatomical elements 

 are traceable through the various forms of the vertebrate 

 skeleton. 

 Generative 3. In the fomis just mentioned, the flower-like genera- 

 organs be- ^^ organs mature their products while in connexion with 



coming o X 



detached their parent stem, as do the flowers of plants. But in 

 e usee, ^^j^^^, nearly aUied forms, the flower-like organs are de- 

 tached before they arrive at maturity, and swim away. 

 They grow to an enormous size in comparison with the 



