No. 514] 



ARE SPECIES HE MAT IE, 



mi 



interest in connection with the problem of the separate- 

 ness of species. Thus two of these groups that outwardly 

 resemble each other the most, and whose distinctness is 

 still doubted by some eminent zoologists, prove, under 

 examination with modern technique, to be possessed of 

 extraordinary histological differences. Hardly may we 

 find in the whole mammalian series corresponding types 

 of cells showing so fundamental differences in structure 

 as are shown by the cnidoblasts of the two species which 

 (since we must choose among the questioned specific 

 titles) are probably to be designated as Hydra fused and 

 H. dicecia. Species, therefore, which Linnams could not 

 distinguish and which it may be hardly possible for the 

 ordinary student to distinguish with the care commonly 

 devoted to such subjects, may yet bo separated by differ- 

 ences which seldom obtain between much more remotely 

 related types. 



Did space permit, I might also illustrate from this 

 genus of organisms the proposition that the integrity, and 

 hence the reality, of species is not destroyed by the fact 

 that certain individuals of one may be transformed into 

 members of another. I will but refer to the fact that it is 

 possible, though extremelv difficult, to transform //. 

 viridis into another, a white Hvdra (H. fusca), differing 

 from it in practically every specific character. I do not 

 refer to the mere bleaching of the green species. This, 

 up to a certain point, is easy, but carries with it no sig- 

 nificant, morphological changes. The transformation 

 which I have effected (in both directions) does carry with 



permanent through an indefinite number of generations 

 and in spite of many environmental changes. 



Of course we may, if we will, degrade these types from 

 specific to varietal 'rank, although thev probably deserve 

 the specific distinction. But the point of emphasis is the 



able to even find them in the same habitat. Interbreeding 



spring respectively) of sexual development. Species, 



