Natural History of the Animalcules, ioi 
these two extremes, Animalcules of all sizes may be met 
with, each kind being limited within a certain range of 
growth, as constant in these lowly organisms, measured 
by their humble fractions of an inch, as it is in the lordly 
beasts, measured by their yards and feet. 
It has already been stated that, in the strift sense of 
the term, Animalcules should only include the most 
minute members of the animal kingdom, and we have 
incidentally seen that these are also the most simple in 
strufture. Among the older naturalists, however, the 
term had a much wider application, for it included not 
only a very large number of minute, highly-organised 
forms, such as allies of the worms, crabs, shell-fish, etc., 
but also a very large number of minute plants. With 
the gradual perfe6lion of the microscope within the last 
fifty years, the highly organised minute animals have 
been weeded out from the Animalcules, and have been 
placed among their allied larger forms ; and a very large 
number of vegetable organisms have shared the same fate. 
Considerable difficulty exists, however, in satisfa6lorily 
dealing with the separation of the minute animal and 
plant forms, for there are many types of life which, in 
certain stages of their existence, so closely resemble 
undoubted animal types that they seem referable to the 
animal kingdom, and which, in other stages, from their 
resemblances to undoubted vegetable forms, seem refera- 
ble to the vegetable kingdom. So much, indeed, was 
this difficulty felt that, a few years ago, it was seriously 
proposed that a kingdom should be formed, intermediate 
between the animal and vegetable kingdoms, for the 
reception of all those doubtful organisms which seemed 
both plants and animals. 
