LETTER XLVII.

_SYSTEM OF INSECTS._

Having considered insects as to their History, Anatomy and Physiology, we must next enter a new and ample field, in which, like most of our predecessors, we shall often be perplexed and bewildered by the infinite variety of devious paths which traverse it, and by the mazy labyrinths in which the more we wander the less ground we seem to gain.--You will easily perceive I am speaking of the _System of Insects_. System is a subject which has engaged the attention of Naturalists from the time of Aristotle to the present day; and even now that it has been so much and so ably discussed, they are far from being agreed concerning it. In our own country a clue has, however, of late been furnished, which upon the whole seems better calculated to enable us to thread the intricate labyrinth of nature, than any thing previously excogitated.

There are two words relating to this subject concerning which Naturalists seem not to have very precise ideas--_Method_ and _System_. They have often been confounded and used indifferently to signify the same thing. Thus we hear of a Natural Method and a Natural System. Linné seems to have regarded the _former_ of these terms as representing the actual disposition of objects in nature[1126], while by _System_ he understands their classification and arrangement by Naturalists[1127]. But if we consider their real meaning,--a _Method_ should signify an _Artificial_, and a _System_ a _Natural_ arrangement of objects[1128]. As many systematists, however, have aimed at giving a _natural_ arrangement, though with various success,--some, as the French school, (to which we are principally indebted for the progress already made,) approximating nearer to the true idea than others,--and none having a _perfect_ conception of it, of which probably in our present state, our minds, from its intricacy, are incapable,--it might perhaps be as well to call every arrangement whose object is confessedly artificial, a _Method_; and that which aims at the plan of nature, a _System_. Under this view system-makers would be divided into two classes,--the _Methodists_ and _Systematists_.

The system of nature, which we are now to consider, may be viewed under a double aspect; for with regard to all created objects there is a _System_ of _Distribution_, and a _System_ of _Correlation_, which appear to be quite independent of each other. The former will best fall under our notice when we are treating of the _Geography_ of insects: I shall therefore now confine myself to the latter.

When the ALMIGHTY CREATOR willed to bring into existence this mundane system, he formed it according to a preconcerted plan, with all its parts beautifully linked together and mutually corresponding. All things were ordered in _measure_, and _number_, and _weight_[1129]. There was nothing deficient, nothing superfluous; but the whole in the strictest sense "was very good[1130]," and calculated in the highest degree to answer the purpose of its GREAT AUTHOR. I call it a system of _Correlation_, because there is discernible in it, in the first place, a concatenation of its parts, by which, as to their forms and uses, objects are linked together in groups by a chain of affinities; so that we pass from one to the other by gentle gradations, without having to overleap any _wide_ interval. We see also a gradual ascent from low to high, from less to more excellent. And this leads us to another kind of relationship between natural objects, by which, though placed in distinct groups or in a different series, they in some sort represent and symbolize each other. Examples of this relationship by analogy are to be found in every kingdom of nature, and often form an ascending series from the lowest to the highest; for, as we shall see hereafter, these resemblances appear to maintain a certain correspondence with each other as to their relative situations; so that, for instance, in the animal kingdom they ascend step by step, without being linked by affinity or having any real juxtaposition, from the lowest groups, towards man, who stands alone at the head, or in the centre of all.--I shall say something on each of these kinds of relationship.

I. The relation of _affinity_ may be considered as to its _series_ and _groups_. A series, of course, consists of parts either _concatenated_ like a chain, or placed _separately_ at small intervals from each other. It may run either in a right line, or deviate from it in various ways. It appears to be the opinion of most modern Physiologists, that the series of affinities in nature is a _concatenated_ or continuous series; and that though an _hiatus_ is here and there observable, this has been caused either by the annihilation of some original group or species in consequence of some great convulsion of nature, or that the objects required to fill it up are still in existence but have not yet been discovered[1131]: and this opinion is founded on a _dictum_ of Linné, _Natura ... saltus non facit_[1132]. If this dictum be liberally interpreted, according to the evident meaning of the word _saltus_, few will be disposed to object to it; since both observation and analogy combine to prove that there must be a regular approximation of things to each other in the works of God; and that could we see the whole according to his original plan, we should find no _violent_ interval to break up that approximation: but if it be contended, that in this plan there is no difference in the juxtaposition of the nearest groups or individuals, and never any interval between them, I think we are going further than either observation or analogy will warrant. Were this really and strictly the case, it seems to follow that every group or individual species must on one side borrow half its characters from the _preceding_ group or species, and on the other impart half to the _succeeding_[1133]. But one of the most evident laws of creation is _variety_; and if we survey all the works of the MOST HIGH, we shall no where discover that kind of order and symmetry that this strict interpretation implies. The general march of nature therefore seems to say, that there must be _varying_ though not _violent_ intervals in the series of beings: or in other words, some _conterminous_ species or groups have more characters in common than others.

It was the opinion of Bonnet (in this field himself a host) and many other Naturalists, that the series of beings was not only continuous, but _undeviating_, ascending in a _direct_ line from the lowest to the highest[1134]. Others, finding that this theory could not be made to accord with the actual state of things in nature, thought that a scale of the kingdoms of nature must represent a _map_ or _net_[1135]; thus abandoning a continuous series: and Lamarck, as was before observed[1136], for the solution of the difficulty, arranged Invertebrate animals in a double subramose one. Mr. W. S. MacLeay and (without consultation nearly at the same time) Professor Agardh, Mr. Fries, &c. have given to the learned world an opinion which approximates more nearly to what we see in nature: viz. That the arrangement of objects is indeed in a continuous series, but which in its progress forms various convolutions, each of which may be represented by a _circle_, or a series that returns into itself[1137]. According to this opinion,--which seems the most consistent of any yet advanced, and which reconciles facts which upon no other plan can be reconciled,--the series of beings is involved in the highest degree, rolling wheel within wheel _ad infinitum_, and revolving, if I may so speak, round its centre and summit--_man_[1138]: who, though not including in himself all that distinguishes them, is still the great Archetype in which they terminate, and from which they degrade on all sides.

It is by this convolving series that the various _groups_ into which the kingdoms of nature seem resolvable are formed. We are instructed by the highest authority that every thing was created "after its kind;" and the common sense of mankind in all ages has imposed classic, generic, and other names implying sections, as well as specific ones, upon natural objects: and though many modern Physiologists have asserted that species form the only _absolute_ division in nature; yet as all seem to allow that there are _groups_, and many that these are represented by a circle or group returning into itself[1139], the most absolute division in nature, we will not contend for a term[1140]. We now come to consider these groups themselves, and may notice them under various denominations.

It is customary to consider all the substances of which our globe consists as divided into _three_ kingdoms,--the _Mineral_, _Vegetable_, and _Animal_; but strictly speaking the _primary_ division is into organized and inorganized matter; the former resolving itself into the two kingdoms last mentioned. These, like England and Scotland of old, have their "Land Debateable;" occupied by those _Productions moyennes_, (to use a term of Bonnet's[1141],) which are as it were partly animal and partly vegetable. From this territory common to both, the two kingdoms are extended in a nearly parallel direction till they reach their extreme limits, without any incursion from either side upon their mutual boundaries, but each showing its kindred with the other by certain resemblances observable between _opposite_ points; so that valley corresponds with valley, mountain with mountain, river with river, sea with sea[1142]; not, however, so as to form an exact counterpart, but only in some general features. But to leave metaphor;--as the vegetable kingdom is distinguished from the mineral by its organization and life, by its circulation of sap, and by its powers of reproduction by seed or otherwise; so is the animal from the vegetable by its powers of volition and locomotion[1143], by its nervous systems and organs of sensation, and the senses to which they minister, by its muscular irritability, and by its instinctive endowments.

Having made these observations with regard to the primary division of natural objects in general,--what I have further to say will be confined to the _animal_ kingdom, and ultimately to the branch of which we are treating.

i. Lamarck divided the animal kingdom into two _provinces_, or _subkingdoms_ as they are now called; the one consisting of all those animals whose skeleton is _internal_ and built upon a vertebral column, which are denominated _Vertebrates_; and the second, of those whose skeleton or its representative is for the most part _external_, including the muscles,--these are called _Invertebrates_[1144]. Though this distinction is so marked as in general to form a most striking characteristic, yet when these two provinces approach each other, it begins to disappear. Thus the vertebral column, forming one piece with the shell[1145], becomes almost _external_ in the Chelonian reptiles, or tortoises and turtles, and almost disappears in the cyclostomous fishes; and there is the beginning of an _internal_ one in the _Cephalopoda_, or cuttle-fish belonging to the Invertebrates. Dr. Virey, assuming the nervous system as his basis, long since divided the animal kingdom, without assigning names to them, into _three_ subkingdoms[1146]; M. Cuvier has _four_--_Vertebrata_; _Mollusca_; _Articulata_; _Radiata_[1147]: and Mr. MacLeay, finding _five_ variations of that system, divides animals into _five_ provinces or subkingdoms, of which I formerly gave you some account[1148];--viz. _Vertebrata_, in which the nervous system has only one principal centre; _Annulosa_, in which it is ganglionic, with the ganglions arranged in a series, with a double spinal chord; _Mollusca_, in which it is ganglionic, with the ganglions dispersed irregularly but connected by nervous threads; _Radiata_, in which it is _filamentous_, with the nervous threads radiating from the mouth; and _Acrita_, in which this system is _molecular_[1149]. And to this division of the kingdom, as founded on a satisfactory basis, I should recommend you to adhere: still however we may speak of vertebrate and invertebrate animals, as forming the _primary_ subdivision of them, taken from a striking character and obvious to every one who sees them.

If you inquire into the rank of each of these subkingdoms, of course you will assign the principal station to the _Vertebrates_, which are the most perfectly organized, to which _man_ belongs, and over which he immediately presides. If we form the scale according to the nervous system of each province, that in which the organ of sensation and intellect is most concentrated will stand first; and in proportion as this organ is multiplied and dispersed will be the station of the rest, which will place them in the order in which I have mentioned them; and the _Annulosa_, to which insects belong, will precede the _Mollusca_, which Cuvier and Lamarck had placed before them on account of their system of circulation. But when we reflect that a _heart_ and _circulation_ occur in some of the conglomerate _Polypi_[1150], animals that approach the _vegetable_ kingdom; that some of the acephalous _Mollusca_ have no visible organs of sense, except that of taste, whose substance is little better than a homogeneous gelatinous pulp, and who seem from their inert nature to have very slight powers of voluntary motion[1151], we shall be convinced that a heart and circulation alone, unaccompanied by a more concentrated nervous system and more perfect structure, cannot place an animal above those which in every other respect so obviously excel them. With regard to _insects_ particularly, we may further ask--Who that considers how man employs his powers and organs even in his most degraded state, or that contemplates the wonderful works that he is enabled to accomplish when his faculties receive their due cultivation and direction, can avoid regarding him as superior to the rest of the animal creation? And what unsophisticated mind, not entangled in the trammels of system, when it surveys the industry, the various proceedings, and almost miraculous works that have been laid before you, the waxen palaces of the bee,--the paper cottages of the wasp and hornet,--the crowded metropolis of the white ants,--the arts, the manufactures, and stratagems of other insects,--the associations and labours for the common good of those that are gregarious;--will not at once conclude that they must be a superior race to the slug, the snail, and others, which live only to eat and propagate their kind?

Or who, that considers the wonderful structure of the animals whose cause I advocate,--the analogy that exists between their organs of manducation, of motion, and of sensation, and between various other parts of it[1152], with those of the higher animals,--the acuteness of their senses, their wonderful strength of muscle[1153], and powers of locomotion[1154],--but will think them superior to the headless and almost inanimate oyster or muscle, or the conglomerate _Alcyonia_, though they have a heart and circulation?

Who again, that observes that in proportion as pedate animals approach to the human type, their motions are accomplished by fewer organs,--that man walks _ore sublimi_ upon _two_ legs; the majority of quadrupeds upon _four_; insects upon _six_: the _Arachnida_ apparently upon _eight_; most _Crustacea_ upon _ten_; and the _Myriapods_ and others upon _many_,--but will thence conclude that insects must precede the _Arachnida_ and _Crustacea_?

Who, once more, that reflects that if any of the superior animals are deprived of a limb it can never be reproduced, and that in insects the same circumstance occurs; while spiders and _Crustacea_ if they lose a leg have the power of reproducing it, and the _Mollusca_ if they are decapitated can gain a new head,--will consent to their being placed after any of these animals[1155]?

Lastly, who that recollects that the _Mollusca_ are hermaphrodites, like most plants, bearing both male and female organs in the same body,--but will allow that insects, in which the sexes are separate as in the Vertebrates, must be more perfect, and of a higher grade[1156]?

ii. We now come to the _Classes_ into which the _Annulosa_ are divided. This term appears first to have been employed by Tournefort, and was adopted by Linné[1157]. As the nervous system of animals furnishes the most prominent distinction of a subkingdom, so the _circulation_ of their fluids, and their _respiration_ necessarily connected with it, seems best to point out the _classes_ into which it may next be resolved. But having fully explained my ideas on this subject in a former letter, I need not here repeat what I then said[1158].

iii. As we have subkingdoms, so we may also have _subclasses_, or such large divisions of a class--not founded upon internal organization or any of the primary vital functions, but upon different modes of taking their food, or such other _secondary_ characters--as include more than one _Order_. To this description Clairville's _Mandibulata_ and _Haustellata_ appear to me to belong, which I think are by no means entitled to the rank of Classes; for whoever compares these two tribes together will at the first glance be convinced, by the numerous characters they possess in common, notwithstanding the different mode in which they take their food, that they form one connected primary group. This circumstance, therefore, only furnishes a clue for their further subdivision into two secondary groups, separated by distinctions certainly of a lower value than those which separate the _Crustacea_ and _Arachnida_ from _Insecta_. This is further confirmed by the variations that take place in their mode of feeding in their different states; some from masticators becoming suctorious (_Lepidoptera_), and others from being suctorious becoming masticators (_Myrmeleon_, _Dytiscus_, &c.),--which shows that this character does not enter the essential idea of the animal.