44 ALTERNATION OF GENERATIONS. 



novel doctrine of " Alternation of Generations." According to 

 this doctrine or hypothesis, the successive stages of advancement 

 through which certain of the lower forms of life pass, are to be 

 regarded, not as different phases of a process of metamorphosis 

 or transformation, like that of the caterpillar into a butterfly, or 

 the tadpole into a frog, but as distinct generations, in which the 

 parent animal produces an offspring totally unlike itself, but 

 which begets a progeny that returns in form and nature to that 

 of the original stock ; so that, as the originator of the idea himself 

 puts it, the maternal animal does not meet with its resemblance 

 in its own brood, but in its descendants of the second, third, or 

 fourth degree of generation. 



The question which this hypothesis opens up is a much larger 

 one than can be properly discussed here ; nor is it possible, 

 perhaps, in the present state of our knowledge, satisfactorily to 

 explain all the facts which relate to it. It must be borne in 

 mind, however, that the facts themselves are already fully 

 established, and that whatever interpretation may be put upon 

 them, they must undoubtedly be ranked amongst the most 

 wonderful phenomena which recent investigations in natural 

 history have brought to light. 



Reference has already been made to the luminosity of some of 

 the Jelly-fish. There is reason to believe that all these animals 

 are endowed with the property of emitting light ; and it is 

 mainly to their agency that we owe the beautiful phenomenon 

 known as " the phosphorescence of the sea." " When on a sum- 

 mer's evening," says a writer before quoted, " the waves flash 

 fire as they break upon the shore, or glow with myriads of sparks 

 us they curl and froth around the prow of the moving ship, or 

 under the blade of the striking oar, it is to delicate and almost 

 invisible Medusa? that they chiefly owe their phophorescence." 

 It has never been clearly ascertained what organs are concerned 

 in the production of this phosphoric light, nor has the most 

 careful examination ever availed to detect the secretion itself, al- 

 though there can be but little doubt that the animals are enabled 

 to exhibit the phenomenon at will. In many of the species, the 

 phosphorescence may be evoked when the animals are kept in 

 confinement. " On being touched or otherwise irritated," says 

 Mr. Gosse, " they suddenly become illuminated, the light ap- 

 pearing in rings or circles of luminous points, which alternately 



