MEANS OF MIGRATION. 355 



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 aU possible 

 localities inaccessible to other animals, and variously modified 



