fad 
DEFINITION OF THE TERM INSECT. 15 
with the details of the-animal kingdom at large to hazard 
any decided opinion upon Mr. MacLeay’s whole system, 
or to ascertain whether all these classes are sufficiently 
distinct?. My sentiments with regard to those of the 
Annulosa I shall state to you hereafter. 
Upon a future occasion I shall consider more at large 
the station to which insects.seem entitled in a system of 
invertebrate animals, which will not accord exactly with 
that assigned by MM. Cuvier and Lamarck. But I am 
now in a field in which I have no intention to expatiate 
further, than as it is connected with the subject of the 
present letter. I shall therefore confine myself in what 
I have more to say, to the definitions of Insecta that have 
been given by modern authors, beginning with that of 
the zoologist last mentioned. Insects form a part of his 
second group, which he terms sensztive animals (animaux 
sensibles), which group he thus defines: ** They are sen- 
tient, but obtain from their sensations only perceptions 
of oljects—a kind of simple ideas which they cannot 
combine to obtain complex ones. Charact. No vertebral 
column ; a brain, and most commonly an elongated me- 
dullary mass ; some distinct senses ; the organs of move- 
ment attached under the skin: form symmetrical, by 
parts, in pairs.” ‘This division of animals, from the 
* The number jive, which Mr. MacLeay assumes for one basis of 
his system as consecrated in Nature, seems to me to yield to the 
number seven, which is consecrated both in Nature and Scripture. 
Metaphysicians reckon seven principal operations of the mind; mu- 
sicians seven principal musical tones; and opticians seven primary 
colours. In Scripture the abstract idea of this number is—comple- 
tion—fullness—perfection. I have a notion, but not yet sufficiently ma- 
tured, that Mr. MacLeay’s quinaries are resolvable into septenaries. 
® Anim. sans Vertebr. i. 381. 
