MEANS OF MIGRATION. 455 
have more easily spread over the whole earth than any other 
animal, and this fact partly explains the extraordinary uni- 
formity of structure which characterizes these two great 
classes of animals. For, although they contain an ex- 
ceedingly large number of different species, and although 
the insect class alone is said to possess more different species 
than all other classes of animals together, yet all the in- 
numerable species of insects, and in like manner, also, the 
different species of birds, agree most strikingly in all 
essential peculiarities of their organization. Hence, in the 
class of insects, as well as in that of birds, we can distinguish 
only a very small number of large natural groups or orders, 
and these few orders differ but very little from one another 
in their internal structure. The orders of birds with their 
numerous species are not nearly as distinct from one another 
as the orders of the mammalian class, containing much fewer 
species ; and the orders of insects, which are extremely rich 
in genera and species, resemble one another much more 
closely in their internal structure than do the much smaller 
orders of the crab class. The general parallelism between 
birds and insects is also very interesting in relation to syste- 
matic zoology; and the great importance of their richness 
in forms, for scientific morphology, lies in the fact that they 
show us how, within the narrowest anatomical sphere, and 
without profound changes of the essential internal organiz- 
ation, the greatest variety in external bodily forms can be 
attained. The reason of this is evidently their flying mode 
of life and their free locomotion. In consequence of this 
birds, as well as insects, have spread very rapidly over 
the whole surface of the earth, have settled in all possible 
localities inaccessible to other animals, and variously modified 
