The Origin of Species

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by Charles Darwin

cells, and much wax would be saved. Again, from the same cause, it would

  be advantageous to the Melipona, if she were to make her cells closer

  together, and more regular in every way than at present; for then, as we

  have seen, the spherical surfaces would wholly disappear, and would all be

  replaced by plane surfaces; and the Melipona would make a comb as perfect

  as that of the hive-bee. Beyond this stage of perfection in architecture,

  natural selection could not lead; for the comb of the hive-bee, as far as

  we can see, is absolutely perfect in economising wax.

  Thus, as I believe, the most wonderful of all known instincts, that of the

  hive-bee, can be explained by natural selection having taken advantage of

  numerous, successive, slight modifications of simpler instincts; natural

  selection having by slow degrees, more and more perfectly, led the bees to

  sweep equal spheres at a given distance from each other in a double layer,

  and to build up and excavate the wax along the planes of intersection. The

  bees, of course, no more knowing that they swept their spheres at one

  particular distance from each other, than they know what are the several

  angles of the hexagonal prisms and of the basal rhombic plates. The motive

  power of the process of natural selection having been economy of wax; that

  individual swarm which wasted least honey in the secretion of wax, having

  succeeded best, and having transmitted by inheritance its newly acquired

  economical instinct to new swarms, which in their turn will have had the

  best chance of succeeding in the struggle for existence.

  No doubt many instincts of very difficult explanation could be opposed to

  the theory of natural selection,--cases, in which we cannot see how an

  instinct could possibly have originated; cases, in which no intermediate

  gradations are known to exist; cases of instinct of apparently such

  trifling importance, that they could hardly have been acted on by natural

  selection; cases of instincts almost identically the same in animals so

  remote in the scale of nature, that we cannot account for their similarity

  by inheritance from a common parent, and must therefore believe that they

  have been acquired by independent acts of natural selection. I will not

  here enter on these several cases, but will confine myself to one special

  difficulty, which at first appeared to me insuperable, and actually fatal

  to my whole theory. I allude to the neuters or sterile females in

  insect-communities: for these neuters often differ widely in instinct and

  in structure from both the males and fertile females, and yet, from being

  sterile, they cannot propagate their kind.

  The subject well deserves to be discussed at great length, but I will here

  take only a single case, that of working or sterile ants. How the workers

  have been rendered sterile is a difficulty; but not much greater than that

  of any other striking modification of structure; for it can be shown that

  some insects and other articulate animals in a state of nature occasionally

  become sterile; and if such insects had been social, and it had been

  profitable to the community that a number should have been annually born

  capable of work, but incapable of procreation, I can see no very great

  difficulty in this being effected by natural selection. But I must pass

  over this preliminary difficulty. The great difficulty lies in the working

  ants differing widely from both the males and the fertile females in

  structure, as in the shape of the thorax and in being destitute of wings

  and sometimes of eyes, and in instinct. As far as instinct alone is

  concerned, the prodigious difference in this respect between the workers

  and the perfect females, would have been far better exemplified by the

  hive-bee. If a working ant or other neuter insect had been an animal in

  the ordinary state, I should have unhesitatingly assumed that all its

  characters had been slowly acquired through natural selection; namely, by

  an individual having been born with some slight profitable modification of

  structure, this being inherited by its offspring, which again varied and

  were again selected, and so onwards. But with the working ant we have an

  insect differing greatly from its parents, yet absolutely sterile; so that

  it could never have transmitted successively acquired modifications of

  structure or instinct to its progeny. It may well be asked how is it

  possible to reconcile this case with the theory of natural selection?

  First, let it be remembered that we have innumerable instances, both in our

  domestic productions and in those in a state of nature, of all sorts of

  differences of structure which have become correlated to certain ages, and

  to either sex. We have differences correlated not only to one sex, but to

  that short period alone when the reproductive system is active, as in the

  nuptial plumage of many birds, and in the hooked jaws of the male salmon.

  We have even slight differences in the horns of different breeds of cattle

  in relation to an artificially imperfect state of the male sex; for oxen of

  certain breeds have longer horns than in other breeds, in comparison with

  the horns of the bulls or cows of these same breeds. Hence I can see no

  real difficulty in any character having become correlated with the sterile

  condition of certain members of insect-communities: the difficulty lies in

  understanding how such correlated modifications of structure could have

  been slowly accumulated by natural selection.

  This difficulty, though appearing insuperable, is lessened, or, as I

  believe, disappears, when it is remembered that selection may be applied to

  the family, as well as to the individual, and may thus gain the desired

  end. Thus, a well-flavoured vegetable is cooked, and the individual is

  destroyed; but the horticulturist sows seeds of the same stock, and

  confidently expects to get nearly the same variety; breeders of cattle wish

  the flesh and fat to be well marbled together; the animal has been

  slaughtered, but the breeder goes with confidence to the same family. I

  have such faith in the powers of selection, that I do not doubt that a

  breed of cattle, always yielding oxen with extraordinarily long horns,

  could be slowly formed by carefully watching which individual bulls and

  cows, when matched, produced oxen with the longest horns; and yet no one ox

  could ever have propagated its kind. Thus I believe it has been with

  social insects: a slight modification of structure, or instinct,

  correlated with the sterile condition of certain members of the community,

  has been advantageous to the community: consequently the fertile males and

  females of the same community flourished, and transmitted to their fertile

  offspring a tendency to produce sterile members having the same

  modification. And I believe that this process has been repeated, until

  that prodigious amount of difference between the fertile and sterile

  females of the same species has been produced, which we see in many social

  insects.

  But we have not as yet touched on the climax of the difficulty; namely, the

  fact that the neuters of several ants differ, not
only from the fertile

  females and males, but from each other, sometimes to an almost incredible

  degree, and are thus divided into two or even three castes. The castes,

  moreover, do not generally graduate into each other, but are perfectly well

  defined; being as distinct from each other, as are any two species of the

  same genus, or rather as any two genera of the same family. Thus in

  Eciton, there are working and soldier neuters, with jaws and instincts

  extraordinarily different: in Cryptocerus, the workers of one caste alone

  carry a wonderful sort of shield on their heads, the use of which is quite

  unknown: in the Mexican Myrmecocystus, the workers of one caste never

  leave the nest; they are fed by the workers of another caste, and they have

  an enormously developed abdomen which secretes a sort of honey, supplying

  the place of that excreted by the aphides, or the domestic cattle as they

  may be called, which our European ants guard or imprison.

  It will indeed be thought that I have an overweening confidence in the

  principle of natural selection, when I do not admit that such wonderful and

  well-established facts at once annihilate my theory. In the simpler case

  of neuter insects all of one caste or of the same kind, which have been

  rendered by natural selection, as I believe to be quite possible, different

  from the fertile males and females,--in this case, we may safely conclude

  from the analogy of ordinary variations, that each successive, slight,

  profitable modification did not probably at first appear in all the

  individual neuters in the same nest, but in a few alone; and that by the

  long-continued selection of the fertile parents which produced most neuters

  with the profitable modification, all the neuters ultimately came to have

  the desired character. On this view we ought occasionally to find

  neuter-insects of the same species, in the same nest, presenting gradations

  of structure; and this we do find, even often, considering how few

  neuter-insects out of Europe have been carefully examined. Mr. F. Smith

  has shown how surprisingly the neuters of several British ants differ from

  each other in size and sometimes in colour; and that the extreme forms can

  sometimes be perfectly linked together by individuals taken out of the same

  nest: I have myself compared perfect gradations of this kind. It often

  happens that the larger or the smaller sized workers are the most numerous;

  or that both large and small are numerous, with those of an intermediate

  size scanty in numbers. Formica flava has larger and smaller workers, with

  some of intermediate size; and, in this species, as Mr. F. Smith has

  observed, the larger workers have simple eyes (ocelli), which though small

  can be plainly distinguished, whereas the smaller workers have their ocelli

  rudimentary. Having carefully dissected several specimens of these

  workers, I can affirm that the eyes are far more rudimentary in the smaller

  workers than can be accounted for merely by their proportionally lesser

  size; and I fully believe, though I dare not assert so positively, that the

  workers of intermediate size have their ocelli in an exactly intermediate

  condition. So that we here have two bodies of sterile workers in the same

  nest, differing not only in size, but in their organs of vision, yet

  connected by some few members in an intermediate condition. I may digress

  by adding, that if the smaller workers had been the most useful to the

  community, and those males and females had been continually selected, which

  produced more and more of the smaller workers, until all the workers had

  come to be in this condition; we should then have had a species of ant with

  neuters very nearly in the same condition with those of Myrmica. For the

  workers of Myrmica have not even rudiments of ocelli, though the male and

  female ants of this genus have well-developed ocelli.

  I may give one other case: so confidently did I expect to find gradations

  in important points of structure between the different castes of neuters in

  the same species, that I gladly availed myself of Mr. F. Smith's offer of

  numerous specimens from the same nest of the driver ant (Anomma) of West

  Africa. The reader will perhaps best appreciate the amount of difference

  in these workers, by my giving not the actual measurements, but a strictly

  accurate illustration: the difference was the same as if we were to see a

  set of workmen building a house of whom many were five feet four inches

  high, and many sixteen feet high; but we must suppose that the larger

  workmen had heads four instead of three times as big as those of the

  smaller men, and jaws nearly five times as big. The jaws, moreover, of the

  working ants of the several sizes differed wonderfully in shape, and in the

  form and number of the teeth. But the important fact for us is, that

  though the workers can be grouped into castes of different sizes, yet they

  graduate insensibly into each other, as does the widely-different structure

  of their jaws. I speak confidently on this latter point, as Mr. Lubbock

  made drawings for me with the camera lucida of the jaws which I had

  dissected from the workers of the several sizes.

  With these facts before me, I believe that natural selection, by acting on

  the fertile parents, could form a species which should regularly produce

  neuters, either all of large size with one form of jaw, or all of small

  size with jaws having a widely different structure; or lastly, and this is

  our climax of difficulty, one set of workers of one size and structure, and

  simultaneously another set of workers of a different size and structure;--a

  graduated series having been first formed, as in the case of the driver

  ant, and then the extreme forms, from being the most useful to the

  community, having been produced in greater and greater numbers through the

  natural selection of the parents which generated them; until none with an

  intermediate structure were produced.

  Thus, as I believe, the wonderful fact of two distinctly defined castes of

  sterile workers existing in the same nest, both widely different from each

  other and from their parents, has originated. We can see how useful their

  production may have been to a social community of insects, on the same

  principle that the division of labour is useful to civilised man. As ants

  work by inherited instincts and by inherited tools or weapons, and not by

  acquired knowledge and manufactured instruments, a perfect division of

  labour could be effected with them only by the workers being sterile; for

  had they been fertile, they would have intercrossed, and their instincts

  and structure would have become blended. And nature has, as I believe,

  effected this admirable division of labour in the communities of ants, by

  the means of natural selection. But I am bound to confess, that, with all

  my faith in this principle, I should never have anticipated that natural

  selection could have been efficient in so high a degree, had not the case

  of these neuter insects convinced me of the fact. I have, therefore,

  discussed this case, at some little but wholly insufficient length, in

 
order to show the power of natural selection, and likewise because this is

  by far the most serious special difficulty, which my theory has

  encountered. The case, also, is very interesting, as it proves that with

  animals, as with plants, any amount of modification in structure can be

  effected by the accumulation of numerous, slight, and as we must call them

  accidental, variations, which are in any manner profitable, without

  exercise or habit having come into play. For no amount of exercise, or

  habit, or volition, in the utterly sterile members of a community could

  possibly have affected the structure or instincts of the fertile members,

  which alone leave descendants. I am surprised that no one has advanced

  this demonstrative case of neuter insects, against the well-known doctrine

  of Lamarck.

  Summary. -- I have endeavoured briefly in this chapter to show that the

  mental qualities of our domestic animals vary, and that the variations are

  inherited. Still more briefly I have attempted to show that instincts vary

  slightly in a state of nature. No one will dispute that instincts are of

  the highest importance to each animal. Therefore I can see no difficulty,

  under changing conditions of life, in natural selection accumulating slight

  modifications of instinct to any extent, in any useful direction. In some

  cases habit or use and disuse have probably come into play. I do not

  pretend that the facts given in this chapter strengthen in any great degree

  my theory; but none of the cases of difficulty, to the best of my judgment,

  annihilate it. On the other hand, the fact that instincts are not always

  absolutely perfect and are liable to mistakes;--that no instinct has been

  produced for the exclusive good of other animals, but that each animal

  takes advantage of the instincts of others;--that the canon in natural

  history, of 'natura non facit saltum' is applicable to instincts as well as

  to corporeal structure, and is plainly explicable on the foregoing views,

  but is otherwise inexplicable,--all tend to corroborate the theory of

  natural selection.

  This theory is, also, strengthened by some few other facts in regard to

  instincts; as by that common case of closely allied, but certainly

  distinct, species, when inhabiting distant parts of the world and living

  under considerably different conditions of life, yet often retaining nearly

  the same instincts. For instance, we can understand on the principle of

  inheritance, how it is that the thrush of South America lines its nest with

  mud, in the same peculiar manner as does our British thrush: how it is

  that the male wrens (Troglodytes) of North America, build 'cock-nests,' to

  roost in, like the males of our distinct Kitty-wrens,--a habit wholly

  unlike that of any other known bird. Finally, it may not be a logical

  deduction, but to my imagination it is far more satisfactory to look at

  such instincts as the young cuckoo ejecting its foster-brothers,--ants

  making slaves,--the larvae of ichneumonidae feeding within the live bodies

  of caterpillars,--not as specially endowed or created instincts, but as

  small consequences of one general law, leading to the advancement of all

  organic beings, namely, multiply, vary, let the strongest live and the

  weakest die.

  Chapter VIII

  Hybridism

  Distinction between the sterility of first crosses and of hybrids --

  Sterility various in degree, not universal, affected by close

  interbreeding, removed by domestication -- Laws governing the sterility of

  hybrids -- Sterility not a special endowment, but incidental on other

  differences -- Causes of the sterility of first crosses and of hybrids --

  Parallelism between the effects of changed conditions of life and crossing

  -- Fertility of varieties when crossed and of their mongrel offspring not

  universal -- Hybrids and mongrels compared independently of their fertility

 

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