98 ACALEPIIS IN GENERAL. Part I. 



species resembling one another very closely, and exliilnting hardly any difterences 

 among themselves, excepting such as arise from age. This identity of the individuals 

 of one and the same species is particularity striking among the Ctenophora). In 

 this order, there are not even sexual difllerences among the individuals, as they are 

 all hermaphrodites. In the Discophora; proper, a somewhat greater diversity pre- 

 vails. In the first place, we notice male and female individuals; and the difference 

 between the sexes is quite striking in some genera, as, for instance, in Aurelia. 

 Next, there occur frequent deviations among them in the normal number of their 

 parts, — their body consisting frequently of one or two spheromeres more than 

 usual, sometimes even of double the normal numl)er, or of a few less. And 

 yet year after year the same Discophora^ reappear upon our shores, with the same 

 range of differences among their individuals. Among Ilydroids, polymorphism pre- 

 vails to a greater or less extent, besides the diflerences arising from sex. Few 

 species have only one kind of individuals. Mostly the cycle of individual differ- 

 ences embraces two distinct types of individuals, one recalling the peculiarities of 

 common Hydrte, the other those of Medusae ; but even the Hydra type of one and 

 the same species may exhibit more or less diversity, there being frequently two 

 kinds of Hydraj united in one and the same community, and sometimes even a 

 larger number of heterogeneous Hydrte. And this is equally true, though to a 

 less extent, of the Medusa type. Yet, among Siphonophora}, there are generally 

 at least two kinds of Medusae in one and the same community. But, notwith- 

 standing this polymorphism among the individuals of one and the same community 

 genetically connected together, each successive generation reproduces the .same kinds 

 of heterogeneous individuals, and nothing but individuals, linked together in the 

 same way. Surely, we have here a much greater diversity of individuals, born 

 one from the other, than is exhibited by the most diversified breeds of our domesti- 

 cated animals ; and yet all these heterogeneous individuals remain true to their 

 species, in one case as in the other, and do not afford the slightest evidence of a 

 transmutation of species. 



Would the supporters of the fanciful theories lately propounded, only extend 

 their studies a little beyond the range of domesticated aninuds, — would they investi- 

 gate the alternate generations of the Acalephs, the extraordinary modes of develop- 

 ment of the Helminths, the reproduction of the Salpa^, etc., etc., — they would soon 

 learn that there are in the world far more astonishing ]3henomena, strictly circum- 

 scribed Ijetween the natural limits of unvarying species, than the slight diflerences 

 produced by the intervention of men, among domesticated animals; and, perhaps, 

 cease to be so confident, as they seem to be, that tliese diflerences are trustworthy 

 indications of the variability of species. For my own part, I must emphatically 

 declare that I do not know a single fact tending to show that species do vary 



