Chap. XVIII.] THE HYDRA. 345 



sporosac, disguised medusa, and attached medusa. On the 

 hypothesis of evolution these forms may be regarded either as 

 (1) stages in the gradual evolution of a perfect medusa, or (2) 

 stages in the degeneration of the perfect medusa, by arrested 

 development owing to changed conditions. The latter view is 

 the more probable. It is difficult to see how the disguised 

 medusa and attached medusa, which have all the parts of the 

 free medusa fitting it for independent existence, but never have 

 an opportunity for employing them, could have arisen by any 

 process of natural selection or otherwise ; or how the possession 

 of a swimming-bell that is never opened could be of any advan- 

 tage to the individual or the species. 



And now for the bearing of these facts on the system of 

 generation in hydra. Here there is not even a sporosac. And 

 it has been recently suggested that in hydra development has 

 been arrested at a yet earlier stage, not only the medusa condi- 

 tion (free, attached, or disguised) having been suppressed, but 

 even the sporosac condition also. This might very possibly be 

 the result of a fresh- water habitat. For it has been shown that 

 fresh-water forms rarely have free and active embryos, since they 

 would tend to be swept out to sea, and this would result in the 

 perishing of the race as a fresh-water type, or in the return of 

 the race to the sea, its primitive home. That power of migration 

 at some stage of its existence, which is so important for the 

 well-being and extension of a marine species, is here abandoned 

 under new conditions of existence. 



If this view of the mode of generation of hydra be correct, 

 this organism must be regarded as a degenerate type. Stellate 

 nerve-cells represent the nervous system. The muscular system 

 is not differentiated. But (on this view) Hydra is the descen- 

 dant of organisms in which nervous system, sense organs, and 

 muscular system were better developed during the medusoid 

 stage. And where a form is descended from organisms more 

 highly developed than itself it is said to be degenerate. For 

 it sometimes happens that an organism is placed in conditions 

 where the possession of organs which were of advantage to its 

 ancestors in the life-struggle are no longer of advantage. 



