The Origin of Species

by Charles Darwin

extremely liable to have their reproductive systems seriously affected.

  This, in fact, is the great bar to the domestication of animals. Between

  the sterility thus superinduced and that of hybrids, there are many points

  of similarity. In both cases the sterility is independent of general

  health, and is often accompanied by excess of size or great luxuriance. In

  both cases, the sterility occurs in various degrees; in both, the male

  element is the most liable to be affected; but sometimes the female more

  than the male. In both, the tendency goes to a certain extent with

  systematic affinity, or whole groups of animals and plants are rendered

  impotent by the same unnatural conditions; and whole groups of species tend

  to produce sterile hybrids. On the other hand, one species in a group will

  sometimes resist great changes of conditions with unimpaired fertility; and

  certain species in a group will produce unusually fertile hybrids. No one

  can tell, till he tries, whether any particular animal will breed under

  confinement or any plant seed freely under culture; nor can he tell, till

  he tries, whether any two species of a genus will produce more or less

  sterile hybrids. Lastly, when organic beings are placed during several

  generations under conditions not natural to them, they are extremely liable

  to vary, which is due, as I believe, to their reproductive systems having

  been specially affected, though in a lesser degree than when sterility

  ensues. So it is with hybrids, for hybrids in successive generations are

  eminently liable to vary, as every experimentalist has observed.

  Thus we see that when organic beings are placed under new and unnatural

  conditions, and when hybrids are produced by the unnatural crossing of two

  species, the reproductive system, independently of the general state of

  health, is affected by sterility in a very similar manner. In the one

  case, the conditions of life have been disturbed, though often in so slight

  a degree as to be inappreciable by us; in the other case, or that of

  hybrids,the external conditions have remained the same, but the

  organisation has been disturbed by two different structures and

  constitutions having been blended into one. For it is scarcely possible

  that two organisations should be compounded into one, without some

  disturbance occurring in the development, or periodical action, or mutual

  relation of the different parts and organs one to another, or to the

  conditions of life. When hybrids are able to breed inter se, they transmit

  to their offspring from generation to generation the same compounded

  organisation, and hence we need not be surprised that their sterility,

  though in some degree variable, rarely diminishes.

  It must, however, be confessed that we cannot understand, excepting on

  vague hypotheses, several facts with respect to the sterility of hybrids;

  for instance, the unequal fertility of hybrids produced from reciprocal

  crosses; or the increased sterility in those hybrids which occasionally and

  exceptionally resemble closely either pure parent. Nor do I pretend that

  the foregoing remarks go to the root of the matter: no explanation is

  offered why an organism, when placed under unnatural conditions, is

  rendered sterile. All that I have attempted to show, is that in two cases,

  in some respects allied, sterility is the common result,--in the one case

  from the conditions of life having been disturbed, in the other case from

  the organisation having been disturbed by two organisations having been

  compounded into one.

  It may seem fanciful, but I suspect that a similar parallelism extends to

  an allied yet very different class of facts. It is an old and almost

  universal belief, founded, I think, on a considerable body of evidence,

  that slight changes in the conditions of life are beneficial to all living

  things. We see this acted on by farmers and gardeners in their frequent

  exchanges of seed, tubers, &c., from one soil or climate to another, and

  back again. During the convalescence of animals, we plainly see that great

  benefit is derived from almost any change in the habits of life. Again,

  both with plants and animals, there is abundant evidence, that a cross

  between very distinct individuals of the same species, that is between

  members of different strains or sub-breeds, gives vigour and fertility to

  the offspring. I believe, indeed, from the facts alluded to in our fourth

  chapter, that a certain amount of crossing is indispensable even with

  hermaphrodites; and that close interbreeding continued during several

  generations between the nearest relations, especially if these be kept

  under the same conditions of life, always induces weakness and sterility in

  the progeny.

  Hence it seems that, on the one hand, slight changes in the conditions of

  life benefit all organic beings, and on the other hand, that slight

  crosses, that is crosses between the males and females of the same species

  which have varied and become slightly different, give vigour and fertility

  to the offspring. But we have seen that greater changes, or changes of a

  particular nature, often render organic beings in some degree sterile; and

  that greater crosses, that is crosses between males and females which have

  become widely or specifically different, produce hybrids which are

  generally sterile in some degree. I cannot persuade myself that this

  parallelism is an accident or an illusion. Both series of facts seem to be

  connected together by some common but unknown bond, which is essentially

  related to the principle of life.

  Fertility of Varieties when crossed, and of their Mongrel off-spring. -- It

  may be urged, as a most forcible argument, that there must be some

  essential distinction between species and varieties, and that there must be

  some error in all the foregoing remarks, inasmuch as varieties, however

  much they may differ from each other in external appearance, cross with

  perfect facility, and yield perfectly fertile offspring. I fully admit

  that this is almost invariably the case. But if we look to varieties

  produced under nature, we are immediately involved in hopeless

  difficulties; for if two hitherto reputed varieties be found in any degree

  sterile together, they are at once ranked by most naturalists as species.

  For instance, the blue and red pimpernel, the primrose and cowslip, which

  are considered by many of our best botanists as varieties, are said by

  Gartner not to be quite fertile when crossed, and he consequently ranks

  them as undoubted species. If we thus argue in a circle, the fertility of

  all varieties produced under nature will assuredly have to be granted.

  If we turn to varieties, produced, or supposed to have been produced, under

  domestication, we are still involved in doubt. For when it is stated, for

  instance, that the German Spitz dog unites more easily than other dogs with

  foxes, or that certain South American indigenous domestic dogs do not

  readily cross with European dogs, the explanation which will occur to

  everyone, and probably the true one, is that these dogs have descended from

  several aboriginally di
stinct species. Nevertheless the perfect fertility

  of so many domestic varieties, differing widely from each other in

  appearance, for instance of the pigeon or of the cabbage, is a remarkable

  fact; more especially when we reflect how many species there are, which,

  though resembling each other most closely, are utterly sterile when

  intercrossed. Several considerations, however, render the fertility of

  domestic varieties less remarkable than at first appears. It can, in the

  first place, be clearly shown that mere external dissimilarity between two

  species does not determine their greater or lesser degree of sterility when

  crossed; and we may apply the same rule to domestic varieties. In the

  second place, some eminent naturalists believe that a long course of

  domestication tends to eliminate sterility in the successive generations of

  hybrids, which were at first only slightly sterile; and if this be so, we

  surely ought not to expect to find sterility both appearing and

  disappearing under nearly the same conditions of life. Lastly, and this

  seems to me by far the most important consideration, new races of animals

  and plants are produced under domestication by man's methodical and

  unconscious power of selection, for his own use and pleasure: he neither

  wishes to select, nor could select, slight differences in the reproductive

  system, or other constitutional differences correlated with the

  reproductive system. He supplies his several varieties with the same food;

  treats them in nearly the same manner, and does not wish to alter their

  general habits of life. Nature acts uniformly and slowly during vast

  periods of time on the whole organisation, in any way which may be for each

  creature's own good; and thus she may, either directly, or more probably

  indirectly, through correlation, modify the reproductive system in the

  several descendants from any one species. Seeing this difference in the

  process of selection, as carried on by man and nature, we need not be

  surprised at some difference in the result.

  I have as yet spoken as if the varieties of the same species were

  invariably fertile when intercrossed. But it seems to me impossible to

  resist the evidence of the existence of a certain amount of sterility in

  the few following cases, which I will briefly abstract. The evidence is at

  least as good as that from which we believe in the sterility of a multitude

  of species. The evidence is, also, derived from hostile witnesses, who in

  all other cases consider fertility and sterility as safe criterions of

  specific distinction. Gartner kept during several years a dwarf kind of

  maize with yellow seeds, and a tall variety with red seeds, growing near

  each other in his garden; and although these plants have separated sexes,

  they never naturally crossed. He then fertilised thirteen flowers of the

  one with the pollen of the other; but only a single head produced any seed,

  and this one head produced only five grains. Manipulation in this case

  could not have been injurious, as the plants have separated sexes. No one,

  I believe, has suspected that these varieties of maize are distinct

  species; and it is important to notice that the hybrid plants thus raised

  were themselves perfectly fertile; so that even Gartner did not venture to

  consider the two varieties as specifically distinct.

  Girou de Buzareingues crossed three varieties of gourd, which like the

  maize has separated sexes, and he asserts that their mutual fertilisation

  is by so much the less easy as their differences are greater. How far

  these experiments may be trusted, I know not; but the forms experimentised

  on, are ranked by Sagaret, who mainly founds his classification by the test

  of infertility, as varieties.

  The following case is far more remarkable, and seems at first quite

  incredible; but it is the result of an astonishing number of experiments

  made during many years on nine species of Verbascum, by so good an observer

  and so hostile a witness, as Gartner: namely, that yellow and white

  varieties of the same species of Verbascum when intercrossed produce less

  seed, than do either coloured varieties when fertilised with pollen from

  their own coloured flowers. Moreover, he asserts that when yellow and

  white varieties of one species are crossed with yellow and white varieties

  of a distinct species, more seed is produced by the crosses between the

  same coloured flowers, than between those which are differently coloured.

  Yet these varieties of Verbascum present no other difference besides the

  mere colour of the flower; and one variety can sometimes be raised from the

  seed of the other.

  From observations which I have made on certain varieties of hollyhock, I am

  inclined to suspect that they present analogous facts.

  Kolreuter, whose accuracy has been confirmed by every subsequent observer,

  has proved the remarkable fact, that one variety of the common tobacco is

  more fertile, when crossed with a widely distinct species, than are the

  other varieties. He experimentised on five forms, which are commonly

  reputed to be varieties, and which he tested by the severest trial, namely,

  by reciprocal crosses, and he found their mongrel offspring perfectly

  fertile. But one of these five varieties, when used either as father or

  mother, and crossed with the Nicotiana glutinosa, always yielded hybrids

  not so sterile as those which were produced from the four other varieties

  when crossed with N. glutinosa. Hence the reproductive system of this one

  variety must have been in some manner and in some degree modified.

  From these facts; from the great difficulty of ascertaining the infertility

  of varieties in a state of nature, for a supposed variety if infertile in

  any degree would generally be ranked as species; from man selecting only

  external characters in the production of the most distinct domestic

  varieties, and from not wishing or being able to produce recondite and

  functional differences in the reproductive system; from these several

  considerations and facts, I do not think that the very general fertility of

  varieties can be proved to be of universal occurrence, or to form a

  fundamental distinction between varieties and species. The general

  fertility of varieties does not seem to me sufficient to overthrow the view

  which I have taken with respect to the very general, but not invariable,

  sterility of first crosses and of hybrids, namely, that it is not a special

  endowment, but is incidental on slowly acquired modifications, more

  especially in the reproductive systems of the forms which are crossed.

  Hybrids and Mongrels compared, independently of their fertility. --

  Independently of the question of fertility, the offspring of species when

  crossed and of varieties when crossed may be compared in several other

  respects. Gartner, whose strong wish was to draw a marked line of

  distinction between species and varieties, could find very few and, as it

  seems to me, quite unimportant differences between the so-called hybrid

  offspring of species, and the so-called mongrel offspring of varieties.

  And, on the other hand, they agree mo
st closely in very many important

  respects.

  I shall here discuss this subject with extreme brevity. The most important

  distinction is, that in the first generation mongrels are more variable

  than hybrids; but Gartner admits that hybrids from species which have long

  been cultivated are often variable in the first generation; and I have

  myself seen striking instances of this fact. Gartner further admits that

  hybrids between very closely allied species are more variable than those

  from very distinct species; and this shows that the difference in the

  degree of variability graduates away. When mongrels and the more fertile

  hybrids are propagated for several generations an extreme amount of

  variability in their offspring is notorious; but some few cases both of

  hybrids and mongrels long retaining uniformity of character could be given.

  The variability, however, in the successive generations of mongrels is,

  perhaps, greater than in hybrids.

  This greater variability of mongrels than of hybrids does not seem to me at

  all surprising. For the parents of mongrels are varieties, and mostly

  domestic varieties (very few experiments having been tried on natural

  varieties), and this implies in most cases that there has been recent

  variability; and therefore we might expect that such variability would

  often continue and be super-added to that arising from the mere act of

  crossing. The slight degree of variability in hybrids from the first cross

  or in the first generation, in contrast with their extreme variability in

  the succeeding generations, is a curious fact and deserves attention. For

  it bears on and corroborates the view which I have taken on the cause of

  ordinary variability; namely, that it is due to the reproductive system

  being eminently sensitive to any change in the conditions of life, being

  thus often rendered either impotent or at least incapable of its proper

  function of producing offspring identical with the parent-form. Now

  hybrids in the first generation are descended from species (excluding those

  long cultivated) which have not had their reproductive systems in any way

  affected, and they are not variable; but hybrids themselves have their

  reproductive systems seriously affected, and their descendants are highly

  variable.

  But to return to our comparison of mongrels and hybrids: Gartner states

  that mongrels are more liable than hybrids to revert to either parent-form;

  but this, if it be true, is certainly only a difference in degree. Gartner

  further insists that when any two species, although most closely allied to

  each other, are crossed with a third species, the hybrids are widely

  different from each other; whereas if two very distinct varieties of one

  species are crossed with another species, the hybrids do not differ much.

  But this conclusion, as far as I can make out, is founded on a single

  experiment; and seems directly opposed to the results of several

  experiments made by Kolreuter.

  These alone are the unimportant differences, which Gartner is able to point

  out, between hybrid and mongrel plants. On the other hand, the resemblance

  in mongrels and in hybrids to their respective parents, more especially in

  hybrids produced from nearly related species, follows according to Gartner

  the same laws. When two species are crossed, one has sometimes a prepotent

  power of impressing its likeness on the hybrid; and so I believe it to be

  with varieties of plants. With animals one variety certainly often has

  this prepotent power over another variety. Hybrid plants produced from a

  reciprocal cross, generally resemble each other closely; and so it is with

  mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced

  to either pure parent-form, by repeated crosses in successive generations

  with either parent.

  These several remarks are apparently applicable to animals; but the subject