The Origin of Species

by Charles Darwin

at least some competent judges.

  That varieties of this doubtful nature are far from uncommon cannot be

  disputed. Compare the several floras of Great Britain, of France or of the

  United States, drawn up by different botanists, and see what a surprising

  number of forms have been ranked by one botanist as good species, and by

  another as mere varieties. Mr. H. C. Watson, to whom I lie under deep

  obligation for assistance of all kinds, has marked for me 182 British

  plants, which are generally considered as varieties, but which have all

  been ranked by botanists as species; and in making this list he has omitted

  many trifling varieties, but which nevertheless have been ranked by some

  botanists as species, and he has entirely omitted several highly

  polymorphic genera. Under genera, including the most polymorphic forms,

  Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112,--a

  difference of 139 doubtful forms! Amongst animals which unite for each

  birth, and which are highly locomotive, doubtful forms, ranked by one

  zoologist as a species and by another as a variety, can rarely be found

  within the same country, but are common in separated areas. How many of

  those birds and insects in North America and Europe, which differ very

  slightly from each other, have been ranked by one eminent naturalist as

  undoubted species, and by another as varieties, or, as they are often

  called, as geographical races! Many years ago, when comparing, and seeing

  others compare, the birds from the separate islands of the Galapagos

  Archipelago, both one with another, and with those from the American

  mainland, I was much struck how entirely vague and arbitrary is the

  distinction between species and varieties. On the islets of the little

  Madeira group there are many insects which are characterized as varieties

  in Mr. Wollaston's admirable work, but which it cannot be doubted would be

  ranked as distinct species by many entomologists. Even Ireland has a few

  animals, now generally regarded as varieties, but which have been ranked as

  species by some zoologists. Several most experienced ornithologists

  consider our British red grouse as only a strongly-marked race of a

  Norwegian species, whereas the greater number rank it as an undoubted

  species peculiar to Great Britain. A wide distance between the homes of

  two doubtful forms leads many naturalists to rank both as distinct species;

  but what distance, it has been well asked, will suffice? if that between

  America and Europe is ample, will that between the Continent and the

  Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be

  admitted that many forms, considered by highly-competent judges as

  varieties, have so perfectly the character of species that they are ranked

  by other highly-competent judges as good and true species. But to discuss

  whether they are rightly called species or varieties, before any definition

  of these terms has been generally accepted, is vainly to beat the air.

  Many of the cases of strongly-marked varieties or doubtful species well

  deserve consideration; for several interesting lines of argument, from

  geographical distribution, analogical variation, hybridism, &c., have been

  brought to bear on the attempt to determine their rank. I will here give

  only a single instance,--the well-known one of the primrose and cowslip, or

  Primula veris and elatior. These plants differ considerably in appearance;

  they have a different flavour and emit a different odour; they flower at

  slightly different periods; they grow in somewhat different stations; they

  ascend mountains to different heights; they have different geographical

  ranges; and lastly, according to very numerous experiments made during

  several years by that most careful observer Gartner, they can be crossed

  only with much difficulty. We could hardly wish for better evidence of the

  two forms being specifically distinct. On the other hand, they are united

  by many intermediate links, and it is very doubtful whether these links are

  hybrids; and there is, as it seems to me, an overwhelming amount of

  experimental evidence, showing that they descend from common parents, and

  consequently must be ranked as varieties.

  Close investigation, in most cases, will bring naturalists to an agreement

  how to rank doubtful forms. Yet it must be confessed, that it is in the

  best-known countries that we find the greatest number of forms of doubtful

  value. I have been struck with the fact, that if any animal or plant in a

  state of nature be highly useful to man, or from any cause closely attract

  his attention, varieties of it will almost universally be found recorded.

  These varieties, moreover, will be often ranked by some authors as species.

  Look at the common oak, how closely it has been studied; yet a German

  author makes more than a dozen species out of forms, which are very

  generally considered as varieties; and in this country the highest

  botanical authorities and practical men can be quoted to show that the

  sessile and pedunculated oaks are either good and distinct species or mere

  varieties.

  When a young naturalist commences the study of a group of organisms quite

  unknown to him, he is at first much perplexed to determine what differences

  to consider as specific, and what as varieties; for he knows nothing of the

  amount and kind of variation to which the group is subject; and this shows,

  at least, how very generally there is some variation. But if he confine

  his attention to one class within one country, he will soon make up his

  mind how to rank most of the doubtful forms. His general tendency will be

  to make many species, for he will become impressed, just like the pigeon or

  poultry-fancier before alluded to, with the amount of difference in the

  forms which he is continually studying; and he has little general knowledge

  of analogical variation in other groups and in other countries, by which to

  correct his first impressions. As he extends the range of his

  observations, he will meet with more cases of difficulty; for he will

  encounter a greater number of closely-allied forms. But if his

  observations be widely extended, he will in the end generally be enabled to

  make up his own mind which to call varieties and which species; but he will

  succeed in this at the expense of admitting much variation,--and the truth

  of this admission will often be disputed by other naturalists. When,

  moreover, he comes to study allied forms brought from countries not now

  continuous, in which case he can hardly hope to find the intermediate links

  between his doubtful forms, he will have to trust almost entirely to

  analogy, and his difficulties will rise to a climax.

  Certainly no clear line of demarcation has as yet been drawn between

  species and sub-species--that is, the forms which in the opinion of some

  naturalists come very near to, but do not quite arrive at the rank of

  species; or, again, between sub-species and well-marked varieties, or

  between lesser varieties and individual differences. These differences

  blend into each other in an insensible series; and a series impresses the


  mind with the idea of an actual passage.

  Hence I look at individual differences, though of small interest to the

  systematist, as of high importance for us, as being the first step towards

  such slight varieties as are barely thought worth recording in works on

  natural history. And I look at varieties which are in any degree more

  distinct and permanent, as steps leading to more strongly marked and more

  permanent varieties; and at these latter, as leading to sub-species, and to

  species. The passage from one stage of difference to another and higher

  stage may be, in some cases, due merely to the long-continued action of

  different physical conditions in two different regions; but I have not much

  faith in this view; and I attribute the passage of a variety, from a state

  in which it differs very slightly from its parent to one in which it

  differs more, to the action of natural selection in accumulating (as will

  hereafter be more fully explained) differences of structure in certain

  definite directions. Hence I believe a well-marked variety may be justly

  called an incipient species; but whether this belief be justifiable must be

  judged of by the general weight of the several facts and views given

  throughout this work.

  It need not be supposed that all varieties or incipient species necessarily

  attain the rank of species. They may whilst in this incipient state become

  extinct, or they may endure as varieties for very long periods, as has been

  shown to be the case by Mr. Wollaston with the varieties of certain fossil

  land-shells in Madeira. If a variety were to flourish so as to exceed in

  numbers the parent species, it would then rank as the species, and the

  species as the variety; or it might come to supplant and exterminate the

  parent species; or both might co-exist, and both rank as independent

  species. But we shall hereafter have to return to this subject.

  From these remarks it will be seen that I look at the term species, as one

  arbitrarily given for the sake of convenience to a set of individuals

  closely resembling each other, and that it does not essentially differ from

  the term variety, which is given to less distinct and more fluctuating

  forms. The term variety, again, in comparison with mere individual

  differences, is also applied arbitrarily, and for mere convenience sake.

  Guided by theoretical considerations, I thought that some interesting

  results might be obtained in regard to the nature and relations of the

  species which vary most, by tabulating all the varieties in several

  well-worked floras. At first this seemed a simple task; but Mr. H. C.

  Watson, to whom I am much indebted for valuable advice and assistance on

  this subject, soon convinced me that there were many difficulties, as did

  subsequently Dr. Hooker, even in stronger terms. I shall reserve for my

  future work the discussion of these difficulties, and the tables themselves

  of the proportional numbers of the varying species. Dr. Hooker permits me

  to add, that after having carefully read my manuscript, and examined the

  tables, he thinks that the following statements are fairly well

  established. The whole subject, however, treated as it necessarily here is

  with much brevity, is rather perplexing, and allusions cannot be avoided to

  the 'struggle for existence,' 'divergence of character,' and other

  questions, hereafter to be discussed.

  Alph. De Candolle and others have shown that plants which have very wide

  ranges generally present varieties; and this might have been expected, as

  they become exposed to diverse physical conditions, and as they come into

  competition (which, as we shall hereafter see, is a far more important

  circumstance) with different sets of organic beings. But my tables further

  show that, in any limited country, the species which are most common, that

  is abound most in individuals, and the species which are most widely

  diffused within their own country (and this is a different consideration

  from wide range, and to a certain extent from commonness), often give rise

  to varieties sufficiently well-marked to have been recorded in botanical

  works. Hence it is the most flourishing, or, as they may be called, the

  dominant species,--those which range widely over the world, are the most

  diffused in their own country, and are the most numerous in

  individuals,--which oftenest produce well-marked varieties, or, as I

  consider them, incipient species. And this, perhaps, might have been

  anticipated; for, as varieties, in order to become in any degree permanent,

  necessarily have to struggle with the other inhabitants of the country, the

  species which are already dominant will be the most likely to yield

  offspring which, though in some slight degree modified, will still inherit

  those advantages that enabled their parents to become dominant over their

  compatriots.

  If the plants inhabiting a country and described in any Flora be divided

  into two equal masses, all those in the larger genera being placed on one

  side, and all those in the smaller genera on the other side, a somewhat

  larger number of the very common and much diffused or dominant species will

  be found on the side of the larger genera. This, again, might have been

  anticipated; for the mere fact of many species of the same genus inhabiting

  any country, shows that there is something in the organic or inorganic

  conditions of that country favourable to the genus; and, consequently, we

  might have expected to have found in the larger genera, or those including

  many species, a large proportional number of dominant species. But so many

  causes tend to obscure this result, that I am surprised that my tables show

  even a small majority on the side of the larger genera. I will here allude

  to only two causes of obscurity. Fresh-water and salt-loving plants have

  generally very wide ranges and are much diffused, but this seems to be

  connected with the nature of the stations inhabited by them, and has little

  or no relation to the size of the genera to which the species belong.

  Again, plants low in the scale of organisation are generally much more

  widely diffused than plants higher in the scale; and here again there is no

  close relation to the size of the genera. The cause of lowly-organised

  plants ranging widely will be discussed in our chapter on geographical

  distribution.

  From looking at species as only strongly-marked and well-defined varieties,

  I was led to anticipate that the species of the larger genera in each

  country would oftener present varieties, than the species of the smaller

  genera; for wherever many closely related species (i.e. species of the same

  genus) have been formed, many varieties or incipient species ought, as a

  general rule, to be now forming. Where many large trees grow, we expect to

  find saplings. Where many species of a genus have been formed through

  variation, circumstances have been favourable for variation; and hence we

  might expect that the circumstances would generally be still favourable to

  variation. On the other hand, if we look at each species as a special act

  of creation, there is no apparent reason
why more varieties should occur in

  a group having many species, than in one having few.

  To test the truth of this anticipation I have arranged the plants of twelve

  countries, and the coleopterous insects of two districts, into two nearly

  equal masses, the species of the larger genera on one side, and those of

  the smaller genera on the other side, and it has invariably proved to be

  the case that a larger proportion of the species on the side of the larger

  genera present varieties, than on the side of the smaller genera.

  Moreover, the species of the large genera which present any varieties,

  invariably present a larger average number of varieties than do the species

  of the small genera. Both these results follow when another division is

  made, and when all the smallest genera, with from only one to four species,

  are absolutely excluded from the tables. These facts are of plain

  signification on the view that species are only strongly marked and

  permanent varieties; for whenever many species of the same genus have been

  formed, or where, if we may use the expression, the manufactory of species

  has been active, we ought generally to find the manufactory still in

  action, more especially as we have every reason to believe the process of

  manufacturing new species to be a slow one. And this certainly is the

  case, if varieties be looked at as incipient species; for my tables clearly

  show as a general rule that, wherever many species of a genus have been

  formed, the species of that genus present a number of varieties, that is of

  incipient species, beyond the average. It is not that all large genera are

  now varying much, and are thus increasing in the number of their species,

  or that no small genera are now varying and increasing; for if this had

  been so, it would have been fatal to my theory; inasmuch as geology plainly

  tells us that small genera have in the lapse of time often increased

  greatly in size; and that large genera have often come to their maxima,

  declined, and disappeared. All that we want to show is, that where many

  species of a genus have been formed, on an average many are still forming;

  and this holds good.

  There are other relations between the species of large genera and their

  recorded varieties which deserve notice. We have seen that there is no

  infallible criterion by which to distinguish species and well-marked

  varieties; and in those cases in which intermediate links have not been

  found between doubtful forms, naturalists are compelled to come to a

  determination by the amount of difference between them, judging by analogy

  whether or not the amount suffices to raise one or both to the rank of

  species. Hence the amount of difference is one very important criterion in

  settling whether two forms should be ranked as species or varieties. Now

  Fries has remarked in regard to plants, and Westwood in regard to insects,

  that in large genera the amount of difference between the species is often

  exceedingly small. I have endeavoured to test this numerically by

  averages, and, as far as my imperfect results go, they always confirm the

  view. I have also consulted some sagacious and most experienced observers,

  and, after deliberation, they concur in this view. In this respect,

  therefore, the species of the larger genera resemble varieties, more than

  do the species of the smaller genera. Or the case may be put in another

  way, and it may be said, that in the larger genera, in which a number of

  varieties or incipient species greater than the average are now

  manufacturing, many of the species already manufactured still to a certain

  extent resemble varieties, for they differ from each other by a less than

  usual amount of difference.

  Moreover, the species of the large genera are related to each other, in the

  same manner as the varieties of any one species are related to each other.