2o6 NATURAL SCIENCE. Sept., 



NEW CONCLUSIONS. 



The position assigned to the Lucernariae (=Calycozoa) has long 

 been that they represent, comparatively but slightly modified, a 

 simply organised ancestral form of those Scyphomedusae called by 

 Haeckel and Lankester the Discomedusae. This view was embodied 

 by L. Agassiz in the comparison of the relation borne by the order 

 to the Discomedusae, to that which the stalked Crinoids (primitive 

 form) bear to the free - moving Comatulidae. This meaning is 

 expressed in the following " tree," which gives the accepted view: — 



Typical Scyphomedusae. Hydromedusae. 



Non-strobilating ancestor ; 

 sessile in early life, free- swimming subsequently. 



Such pedigree has the excellent advantage of simplicity — a lowly- 

 organised animal being placed as the common ancestor both of com- 

 plex and of, presumably, little changed, simply organised descendants. 



Quite recently some slight observations (4), which I recorded in 

 the July number of Natural Science (p. 33), decided me to examine 

 afresh into the truth of this conclusion, with the result that new 

 evidence points distinctly to a completely different phylogenetic 

 arrangement, whereby the Lucernariae would appear to owe origin to 

 an ancestor having as complex a life-cycle as, say, such a typical 

 Discomedusid as Aurelia has at the present day, and not to a simple 

 non-strobilating scyphistomatous type of animal. 



This origin, I believe, came about by the premature develop- 

 ment of genital products during the scyphistoma stage of the ancestral 

 form. When such hastening of events took place — ova and sperm 

 being formed and set free prior to strobilation — the latter phase 

 would in consequence be rendered abortive, no ephyrae would be 

 thrown off, and no medusid stage would occur. Such a change in 

 the life-cycle would clearly be advantageous, as under sessile con- 

 ditions of life competition with free-swimming medusae would be 

 avoided, and thus the variation resulting from premature sexual 

 maturity would tend to be repeated, and finally to be perpetuated. 

 This same avoidance of competition with the Discomedusae accounts 

 for the non-operation of that most general law of Natural Selection 

 — that when through advantageous variation a new species is 

 evolved, the old or stock species is forced from the field and becomes 



