Miscellaneous . 231 



There is another feature of the species of Acalephs which deserves 

 particularly to be noticed. All these animals are periodical in their 

 appearance, and last for a short period in their perfect state of 

 development. In our latitude most Medusae make their appearance 

 as Ephyrse, early in the spring, and rapidly enlarge to their full size. 

 In September and October they lay their eggs, and disappear ; the 

 young hatched from the eggs move about, as Planulae, for a short 

 time, and then become attached, as Scyphistomes, and pass the 

 winter in undergoing their Strobila metamorphosis. The Ctenophorse 

 appear also very early, and lay their eggs in the autumn, passing the 

 winter as young, and growing to their full size towards the beginning 

 of the summer. Among the Hydroids there is more diversity in 

 their periodicity. Hydraria are found all the year round ; but the 

 Medusa-buds, the free Medusae, and the Medusaria make their ap- 

 pearance in different seasons in different species. Some bring forth 

 Medusa-buds and free Medusae or Medusaria during winter ; others 

 (and, in our latitude, this is the case with by far the largest number 

 of the Hydroids) produce their Medusa-brood in the spring ; a few 

 breed later, in the summer or in the autumn ; so that, notwithstand- 

 ing the regularity of their periodical return, Acalephs may be studied, 

 in some condition or other, during the whole year. 



When considering Individuality and Specific Differences as mani- 

 fested in the class of Acalephs, I have taken an opportunity of show- 

 ing, upon general grounds, how futile the arguments are upon which 

 the theory of transmutation of species is founded. Having now 

 shown that that class is circumscribed within definite limits, I may 

 be per-mitted to add here a few more objections to that theory, based 

 chiefly upon special grounds connected with the characteristics of 

 classes. If there is anything striking in the features which distin- 

 guish classes, it is the definiteness of their structural peculiarities ; 

 and this definiteness goes on increasing, with new and additional 

 qualifications, as we pass from the class characters to those which 

 mark the orders, the families, the genera, and the species. Grant- 

 ing, for the sake of argument, that organized beings living at a later 

 period may have originated by a gradual change of those of earlier 

 periods, one of the most characteristic features of all organized beings 

 remains totally unexplained by the various theories brought forward 

 to explain that change — the definiteness of their respective groups, 

 be they ever so comprehensive or ever so limited, combined with the 

 greatest inequality in their numeric relations. There exist a few 

 thousand Mammalia and Reptiles, and at least three times their 

 number of Birds and Fishes. There may be twenty thousand Mol- 

 lusks ; but there are over a hundred thousand Insects, and only a few 

 thousand Radiates. And yet the limits of the class of Insects are as 

 well defined as those of any other class, with the single exception of the 

 class of Birds, which is unquestionably the most definite in its natural 

 boundaries. Now, the supporters of the transmutation theory may 

 shape their views in whatever way they please to suit the require- 

 ments of the theory instead of building the theory upon the facts of 

 Nature ; they never can make it appear that the definiteness of the 

 characters of the class of Birds is the result of a common descent of 



