Heredity and Natural Selection. 305 



two larval forms liave diverG^ccl in two directions from 

 this ancestor, from wliicli tliev inherit all that tliej have 

 in common. 



Ilaeckel believes that after this separation took place, 

 the veiled meduss3 were developed from hydroid po]yi)S, 

 while the veilless medusa were developed from scyphis- 

 toma polyps. Tlie many points of resemblance between 

 the two forms of medusce are, therefore, not due to 

 common inheritance, but have been secondarily uc- 

 cpiired. They are due to the fact that the two groups of 

 medusae have been evolved along parallel but distinct 

 lines. 



Haeckers familiarity with the medusce entitles him to 

 speak with great authority; but still he may possibly be 

 wrong, and the origin of the two groups may not have 

 been as he supposes. 



There are four possible hypotheses as to the origin 

 of the medusae, in favor of each of which something may 

 be said. We may hold with Haeckel that the two larval 

 polyps are the divergent descendants of a common an- 

 cestral polyp, which had no medusa stage, and that 

 each has subsequently developed medusae, or we may be- 

 lieve that the common ancestor Avas a medusa without 

 a polyp larva, and that the hydra larva and the scy- 

 phistoma larva have been independently acquired, or 

 we may believe that the ancestral form had both a lar- 

 val polyp-like stage, and an adult medusa stage, or fin- 

 ally we may assume what seems to us the most probable 

 view, that the ancestral form was neither a true swim- 

 ming medusa nor a true sedentary polyp, but some- 

 thing half-way between, like the actinula of Tubularia 

 or the embryo of Cunina. I do not see any fifth al- 

 ternative, and one of these four suppositions must cor- 

 respond with the actual evolution of the group. Now 



