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 Medusee make their appearance 
as Ephyre, 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 Planule, for a short 
time, and then become attached, as Scyphistomes, and pass the 
winter in undergoing their Strobila metamorphosis. The Ctenophorze 
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 Medusze, and the Medusaria make their ap- 
rance in different seasons in different species. Some bring forth 
Medusa-buds and free Medusee 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 permitted 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 
