350 



NATURE 



[Fed. 29, 1872 



WALLACE ON THE ORIGIN OF INSECTS* 



AMID all the discussions to which the question of the Origin 

 of Insects has given rise, it is to me surprising that one of 

 the most ingenious and remarkable theories ever put forth on a 

 question of natural history has not been so much as once alluded 

 to. More than six years ago, Mr. Herbert Spencer published, 

 in his " Principles of Biology," a vievir of the nature and origin 

 of the annulose type of animals, which goes to the very root of 

 the whole question ; and, if this view is a sound one, it must so 

 materially affect the interpretation of all embryological and 

 anatomical facts bearing on this great subject, that those who 

 work in ignorance of it can hardly hope to arrive at true results. 

 I propose, therefore, to lay before you a brief sketch of Mr. 

 Spencer's tlieory, with the hope of calling attention to it, and 

 inducing some of you to take up what seems to me to be a most 

 promising line of research ; and, although the question is one 

 on which I feel quite incompetent to form a sound judgment, I 

 shall call your attention to the light which it seems to throw on 

 some of the most curious anomalies of insect structure. 



The theory itself may be enunciated in very few worr's. It is, 

 that insects, as well as all the Armulosa, are not primarily single 

 individuals, but that each one is a compound, representing as 

 many individuals as there are true segments in the body, these 

 individuals having become severally differentiated and specialised 

 to perform certain definite functions for the good of the whole 

 compound animal. 



Mr. Spencer first calls attention to the fact, that among the 

 undoubtedly compound animals (which are almost all found in 

 the sub-kingdoms, Ccelenterata and Molluscoida) the several 

 individuals are rarely combined in such a manner as to necessitate 

 any physiological division of labour amoug them. The associated 

 individuals of aHydrozoonor an Ascidian are each free to spread 

 their tentacles, to draw in currents of water, and to select their 

 food, without in any way interfering with each other, because 

 the compound animal is either branched or approximately hemi- 

 spherical, and thus there is no necessity for any of the combined 

 individuals to become especially modified with regard to the rest. 

 But should a compound animal have its component individuals 

 arranged in a linear series, there would most probably arise a 

 marked difference of conditions between the two situated at the 

 extrc.aities and those between them. If they remained united, 

 some modification must have occurred to adapt each to its condi- 

 tion. But if, further, the series should be fixed at one end, the 

 other being free, a new difterentiation must arise ; for the two 

 ends being very differently situated, the intermediate ones will 

 also differ accordingly as they are nearer one end or the other. 

 Here there is a cause for the differentiation of united individuals 

 that does not exist in any branched or other symmetrical arrange- 

 ment than a linear one. Some of the Salpida' show such a 

 rudimentary linear aggregation, but their mouths and vents being 

 lateral the individuals are so similarly situated that no differenti- 

 ation need occur. A little consideration will show us that this 

 is one of those cases in which perfectly transitional forms are not 

 to be expected. A permanent union of individuals in a linear 

 series, such as to necessitate differentiation of function among 

 them, could only be effected by a series of co-ordinated grada- 

 tions, each of which would have so great an advantage over its 

 predecessor as to necessitate its extinction in the struggle for 

 existence. We cannot expect to find the union without the 

 differentiation, or the differentiation without the complete union ; 

 and it will, therefore, be impossible to prove that such was the 

 origin of any group of animals, except by showing tliat numerous 

 traces of separate individualities occur in their organisation, and 

 cannot be explained by any of the known laws of development 

 or growth in animals not so compounded. 



In the structure of the lower Annelids we do find strong indi- 

 cations of such an ancestral fusion of distinct individuals. These 

 animals are composed of segments, not merely superficial, but 

 exhibiting throughout a wonderful identity of form and structure. 

 Each segment has its branchia;, its enlargement of the alimentary 

 canal, its contractile dilatation of the great blood-vessel, its 

 ganglia, its branches from the nervous and vascular trunks, its 

 organs of reproduclion, its locomotive appendages, and, some- 

 times, even its pair of eyes. Thus every segment is a physio- 

 logical whole, having all the organs essential to life and 

 multiplication. Again, just as other compound animals increase 

 by gemmation or fission, so do these. The embryo leaves the 



* Extracted from an Address read at the Anniversary IMeeting of the Ento- 

 molo^cal Society of London on the 22nd January, 1872, by Altired R. 

 ■lyallace, F,L.S., F.Z.S., President, &c 



egg a globular ciliated gemmule ; elongation and segmentation 

 then take place, always in the hinder part, so as to elongate the 

 compound animal without interfering with the more specialised 

 anterior segment. In the Neraertida;, and some Planaria, 

 spontaneous fission occurs, each part becoming a perfect animal, 

 and in the Taenia this is the usual mode of reproduction. The 

 account given by Professor Owen in his " Comparative Anatomy 

 of Invertebrater " is very suggestive of Mr. Spencer's view. He 

 says :^" On the first appearance of the embryo annelid it usually 

 consists of a single segment, which is chiefly occupied by a large 

 mass of unmetamorphosed germ-cells. And these are not used 

 up, as in higher animals, in developing the tissues and organs of 

 an undivided or individual whole, but, after a comparatively 

 slight growth and change of the primary segment, proceed in the 

 typical orders to form a second segment of somewhat simpler 

 structure, and then repeat such formations in a linear series, 

 perhaps more than a hundred times. So that we may have a 

 seeming individual annelid, consisting of many hundred segments, 

 in which a single segment would give all the characteristic 

 organisation of such individual, except some slight additions or 

 modifications, characterising the first and last of the series." 

 lie also tells us that spontaneous fission has now been observed 

 to take place in almost every order of Annulata ; and, in many, 

 artificial fission produces two distinct individuals. In some cases 

 the compound animal consists of very few segments, three only 

 in the genus Chajtogaster, the fourth always separating as a zooid, 

 and forming a new animal. In the higher Articulata, the process 

 of gemmation goes on to a considerable ^extent in the egg, and 

 even afterwards in some cases, but more or less irregularly. 

 Thus the larva of lulus is hatched with eight segments, and at 

 the first moult it acquires si.x new ones, which are added between 

 the last and the penultimate. 



The gradual fusion of the once distinct individuals into a 

 complete unity, is shown in a very interesting manner as we 

 advance from the lower to the higher fomis. In the Annelida, 

 Dr. Carjienter tells us, the spiracles of each segment are separate, 

 and do not communicate internally wiih those of other segments. 

 In the Myriapoda they partially communicate, while in the 

 Insecta they communicate perfectly by a system of anastomosing 

 vessels. The same thing is indicated by the various positions of 

 the chief spiracles. In Smynthurus among the Poduridaa there 

 are only two, opening under the side of the head immediately 

 beneath the antennae. In Solpugida; (Arachnida) they are situated 

 between the anterior feet ; in some spiders they open near the 

 end of the abdomen, in others at its base. The position of the 

 mouth and eyes at the anterior extremity of the body, and the 

 vent at the posterior, are obviously what would arise as soon as 

 any specialisation of function in the series of zooids occurred. 

 It is not, therefore, surprising that we never find these change 

 their position. But for the respiratory and generative organs 

 there is no such necessity for fixity of position, and as ikcy 

 existed originally in every segment, we can well conceive how, 

 as articulate forms become more and more modified, it would 

 sometimes be useful to the compound animal for these organs to 

 become abortive or developed in different parts of the body. 

 We have seen that this is to some extent the case with the 

 former organs, but it occurs to a much greater extent with the 

 latter. 



The most generalised form is to be seen in the intestinal 

 worms, each segment of which possesses a complete hermaphro- 

 dite reproductive apparatus ; so that, in this respect, no less than 

 in their capacity for spontaneous fission, these creatures are 

 really what we shoidd expect the early type of compound 

 animals to be. This, however, is a rare case, but even in the 

 much higher leeches there are testes in no less than nine of the 

 segments, and Dr. Williams discovered a du'ect passage from 

 the spermatheca to the ovaries, which seems to indicate internal 

 self-fertilisation. It is, however, in the lower Arthropoda that 

 we find the most curious diversities in the position of these 

 organs. In the Glomerida; the genital openings in both sexes are 

 situated in the third segment, just behind the insertion of the 

 second pair of limbs. In the Polydesmidce the female organs 

 are in the third segment, while those of the male are in the 

 seventh segment. In lulus the same organs are situated in the 

 fourth and seventh segments respectively. The Chilopoda, on 

 the other hand, have them near the end of the body, as in most 

 insects. In the Acarina the ovaries open on the middle ot the 

 abdomen or on the under side of the thorax, either between or 

 behind the last pair of legs. In spiders the seminal orifice is at 

 the tase of the abdomen, but the palpi are the intromittent 



