60 THE ENTOMOLOGIST. 
before was obscure, and have pointed out some striking 
resemblances, or affinities, as it is the fashion to call them. 
You have proved to my satisfaction the centrality of certain 
groups or types of form containing some of the charac- 
teristics of the surrounding groups, together with a character 
peculiarly their own. This, it appears to me, must be the 
key to affinities, if such exist. ‘That there are really seven 
great and perfectly natural groups of insects, and that they 
approach each other as you have represented, appears 
undeniable. Divide any one of them, and the parts lose 
their relative value when compared with the other groups.” 
—Extract from a letter from Dr. Harris to LE. Newman, 
dated January 7th, 1844; and published in the Memoir 
of Dr. Harris, by Col. T. W. Higginson, prefixed to the 
Entomological Correspondence of 1. W.. Harris, edited by 
Samuel H. Scudder, 1869. 
At the risk of being considered prosy in the repetition of a 
thrice-told tale, I will repeat Cuvier’s “ distribution of animals 
according to their organisation,” and define four groups, 
which, though virtually identical with those I am about to 
employ, have different names. The divisions are these :— 
1. Endosteate animals, having an endo-skeleton, or 
internal framework of bone, to which the muscles are 
attached; the muscles clothe and cover the endo-skeleton, 
and both are enclosed in a sack, called the skin. We are 
told by anatomists that this endo-skeleton is continually 
undergoing disintegration, absorption, and renewal; but of 
this I am incapable of forming an opinion, still less can I 
describe any portion of the process. Nevertheless, seeing 
that the exo-skeleton of the next group is_ repeatedly 
discarded and reproduced, I am perfectly ready to admit 
an analogous phenomenon may exist in the endo-skeleton, 
although the process by which it is performed is so widely 
different that one fails to follow it in all its details. [These 
are the Vertebrata of Cuvier. ] 
2. LHawosteate animals, which have no internal frame- 
work of bone, but, in its stead, an indurated skin, enveloping 
and enclosing the softer parts; and this I call the exo- 
skeleton, or external skeleton. This answers the same purpose 
of protection and support to the muscles as the endo-skeleton, 
but its position is exactly the reverse. ‘The exo-skeleton, as 
