EVOLUTION AND DEGENERATION OF ORGANS 25 



the development consists in an increasing com- 

 plexity of the cell with no formation of unnecessary 

 rudimentary parts. 



3. Phylogenetic evolution of function. Evolution 

 may be regarded from a physiological, as well as 

 from an anatomical standpoint, 1 but, in the former 

 case, evolution is less a set of changes of function 

 than an increasing specialization and division of 

 labour, and under these circumstances it is often diffi- 

 cult to recognize a degenerative element in the evolu- 

 tion. A few examples will demonstrate this point : 



Self-mutilation is a very common phenomenon 

 among Echinoderms. Among brittle star-fish this 

 reaction is controlled by some region of the nervous 

 system ; in some star-fish the reaction follows more 

 quickly because the stimulus can act upon the 

 ganglion at the root of each arm near the circum- 

 oral nerve-collar. In AsteracantJium rubens, there 

 is a complete localization of this function, and self- 

 mutilization only results when an exact region of 

 the nervous system is stimulated. 2 In the Medusa 

 we find an equally interesting example of functional 

 evolution (Eomanes). 3 With some of these (the 



1 Evolution fonctionnelle du systeme nerveux. J. Demoor, Revue 

 universitaire, Bruxelles, 1892. 



2 Contribution & la physiologie nerveuse des Echinodermes. J. 

 Deraoor and M. Chapeaux. Tijds. Ned. DierTc. Vereen. (2) III. 

 2 Nov. 1891. 



3 Preliminary Observations on the Locomotor System of Medusa ; 

 Jelly-fish, Star-fish, and Sea-urchins. Romanes, Int. Scient. Series, 

 1885, 



