The Origin of Species

Home > Science > The Origin of Species > Page 14
The Origin of Species Page 14

by Charles Darwin

and are thus enabled to encroach on places occupied by other beings. Now

  let us see how this principle of great benefit being derived from

  divergence of character, combined with the principles of natural selection

  and of extinction, will tend to act.

  The accompanying diagram will aid us in understanding this rather

  perplexing subject. Let A to L represent the species of a genus large in

  its own country; these species are supposed to resemble each other in

  unequal degrees, as is so generally the case in nature, and as is

  represented in the diagram by the letters standing at unequal distances. I

  have said a large genus, because we have seen in the second chapter, that

  on an average more of the species of large genera vary than of small

  genera; and the varying species of the large genera present a greater

  number of varieties. We have, also, seen that the species, which are the

  commonest and the most widely-diffused, vary more than rare species with

  restricted ranges. Let (A) be a common, widely-diffused, and varying

  species, belonging to a genus large in its own country. The little fan of

  diverging dotted lines of unequal lengths proceeding from (A), may

  represent its varying offspring. The variations are supposed to be

  extremely slight, but of the most diversified nature; they are not supposed

  all to appear simultaneously, but often after long intervals of time; nor

  are they all supposed to endure for equal periods. Only those variations

  which are in some way profitable will be preserved or naturally selected.

  And here the importance of the principle of benefit being derived from

  divergence of character comes in; for this will generally lead to the most

  different or divergent variations (represented by the outer dotted lines)

  being preserved and accumulated by natural selection. When a dotted line

  reaches one of the horizontal lines, and is there marked by a small

  numbered letter, a sufficient amount of variation is supposed to have been

  accumulated to have formed a fairly well-marked variety, such as would be

  thought worthy of record in a systematic work.

  The intervals between the horizontal lines in the diagram, may represent

  each a thousand generations; but it would have been better if each had

  represented ten thousand generations. After a thousand generations,

  species (A) is supposed to have produced two fairly well-marked varieties,

  namely a1 and m1. These two varieties will generally continue to be

  exposed to the same conditions which made their parents variable, and the

  tendency to variability is in itself hereditary, consequently they will

  tend to vary, and generally to vary in nearly the same manner as their

  parents varied. Moreover, these two varieties, being only slightly

  modified forms, will tend to inherit those advantages which made their

  common parent (A) more numerous than most of the other inhabitants of the

  same country; they will likewise partake of those more general advantages

  which made the genus to which the parent-species belonged, a large genus in

  its own country. And these circumstances we know to be favourable to the

  production of new varieties.

  If, then, these two varieties be variable, the most divergent of their

  variations will generally be preserved during the next thousand

  generations. And after this interval, variety a1 is supposed in the

  diagram to have produced variety a2, which will, owing to the principle of

  divergence, differ more from (A) than did variety a1. Variety m1 is

  supposed to have produced two varieties, namely m2 and s2, differing from

  each other, and more considerably from their common parent (A). We may

  continue the process by similar steps for any length of time; some of the

  varieties, after each thousand generations, producing only a single

  variety, but in a more and more modified condition, some producing two or

  three varieties, and some failing to produce any. Thus the varieties or

  modified descendants, proceeding from the common parent (A), will generally

  go on increasing in number and diverging in character. In the diagram the

  process is represented up to the ten-thousandth generation, and under a

  condensed and simplified form up to the fourteen-thousandth generation.

  But I must here remark that I do not suppose that the process ever goes on

  so regularly as is represented in the diagram, though in itself made

  somewhat irregular. I am far from thinking that the most divergent

  varieties will invariably prevail and multiply: a medium form may often

  long endure, and may or may not produce more than one modified descendant;

  for natural selection will always act according to the nature of the places

  which are either unoccupied or not perfectly occupied by other beings; and

  this will depend on infinitely complex relations. But as a general rule,

  the more diversified in structure the descendants from any one species can

  be rendered, the more places they will be enabled to seize on, and the more

  their modified progeny will be increased. In our diagram the line of

  succession is broken at regular intervals by small numbered letters marking

  the successive forms which have become sufficiently distinct to be recorded

  as varieties. But these breaks are imaginary, and might have been inserted

  anywhere, after intervals long enough to have allowed the accumulation of a

  considerable amount of divergent variation.

  As all the modified descendants from a common and widely-diffused species,

  belonging to a large genus, will tend to partake of the same advantages

  which made their parent successful in life, they will generally go on

  multiplying in number as well as diverging in character: this is

  represented in the diagram by the several divergent branches proceeding

  from (A). The modified offspring from the later and more highly improved

  branches in the lines of descent, will, it is probable, often take the

  place of, and so destroy, the earlier and less improved branches: this is

  represented in the diagram by some of the lower branches not reaching to

  the upper horizontal lines. In some cases I do not doubt that the process

  of modification will be confined to a single line of descent, and the

  number of the descendants will not be increased; although the amount of

  divergent modification may have been increased in the successive

  generations. This case would be represented in the diagram, if all the

  lines proceeding from (A) were removed, excepting that from a1 to a10. In

  the same way, for instance, the English race-horse and English pointer have

  apparently both gone on slowly diverging in character from their original

  stocks, without either having given off any fresh branches or races.

  After ten thousand generations, species (A) is supposed to have produced

  three forms, a10, f10, and m10, which, from having diverged in character

  during the successive generations, will have come to differ largely, but

  perhaps unequally, from each other and from their common parent. If we

  suppose the amount of change between each horizontal line in our diagram to

  be excessively small, these three forms may still be only well-marked


  varieties; or they may have arrived at the doubtful category of

  sub-species; but we have only to suppose the steps in the process of

  modification to be more numerous or greater in amount, to convert these

  three forms into well-defined species: thus the diagram illustrates the

  steps by which the small differences distinguishing varieties are increased

  into the larger differences distinguishing species. By continuing the same

  process for a greater number of generations (as shown in the diagram in a

  condensed and simplified manner), we get eight species, marked by the

  letters between a14 and m14, all descended from (A). Thus, as I believe,

  species are multiplied and genera are formed.

  In a large genus it is probable that more than one species would vary. In

  the diagram I have assumed that a second species (I) has produced, by

  analogous steps, after ten thousand generations, either two well-marked

  varieties (w10 and z10) or two species, according to the amount of change

  supposed to be represented between the horizontal lines. After fourteen

  thousand generations, six new species, marked by the letters n14 to z14,

  are supposed to have been produced. In each genus, the species, which are

  already extremely different in character, will generally tend to produce

  the greatest number of modified descendants; for these will have the best

  chance of filling new and widely different places in the polity of nature:

  hence in the diagram I have chosen the extreme species (A), and the nearly

  extreme species (I), as those which have largely varied, and have given

  rise to new varieties and species. The other nine species (marked by

  capital letters) of our original genus, may for a long period continue

  transmitting unaltered descendants; and this is shown in the diagram by the

  dotted lines not prolonged far upwards from want of space.

  But during the process of modification, represented in the diagram, another

  of our principles, namely that of extinction, will have played an important

  part. As in each fully stocked country natural selection necessarily acts

  by the selected form having some advantage in the struggle for life over

  other forms, there will be a constant tendency in the improved descendants

  of any one species to supplant and exterminate in each stage of descent

  their predecessors and their original parent. For it should be remembered

  that the competition will generally be most severe between those forms

  which are most nearly related to each other in habits, constitution, and

  structure. Hence all the intermediate forms between the earlier and later

  states, that is between the less and more improved state of a species, as

  well as the original parent-species itself, will generally tend to become

  extinct. So it probably will be with many whole collateral lines of

  descent, which will be conquered by later and improved lines of descent.

  If, however, the modified offspring of a species get into some distinct

  country, or become quickly adapted to some quite new station, in which

  child and parent do not come into competition, both may continue to exist.

  If then our diagram be assumed to represent a considerable amount of

  modification, species (A) and all the earlier varieties will have become

  extinct, having been replaced by eight new species (a14 to m14); and (I)

  will have been replaced by six (n14 to z14) new species.

  But we may go further than this. The original species of our genus were

  supposed to resemble each other in unequal degrees, as is so generally the

  case in nature; species (A) being more nearly related to B, C, and D, than

  to the other species; and species (I) more to G, H, K, L, than to the

  others. These two species (A) and (I), were also supposed to be very

  common and widely diffused species, so that they must originally have had

  some advantage over most of the other species of the genus. Their modified

  descendants, fourteen in number at the fourteen-thousandth generation, will

  probably have inherited some of the same advantages: they have also been

  modified and improved in a diversified manner at each stage of descent, so

  as to have become adapted to many related places in the natural economy of

  their country. It seems, therefore, to me extremely probable that they

  will have taken the places of, and thus exterminated, not only their

  parents (A) and (I), but likewise some of the original species which were

  most nearly related to their parents. Hence very few of the original

  species will have transmitted offspring to the fourteen-thousandth

  generation. We may suppose that only one (F), of the two species which

  were least closely related to the other nine original species, has

  transmitted descendants to this late stage of descent.

  The new species in our diagram descended from the original eleven species,

  will now be fifteen in number. Owing to the divergent tendency of natural

  selection, the extreme amount of difference in character between species

  a14 and z14 will be much greater than that between the most different of

  the original eleven species. The new species, moreover, will be allied to

  each other in a widely different manner. Of the eight descendants from (A)

  the three marked a14, q14, p14, will be nearly related from having recently

  branched off from a10; b14 and f14, from having diverged at an earlier

  period from a5, will be in some degree distinct from the three first-named

  species; and lastly, o14, e14, and m14, will be nearly related one to the

  other, but from having diverged at the first commencement of the process of

  modification, will be widely different from the other five species, and may

  constitute a sub-genus or even a distinct genus.

  The six descendants from (I) will form two sub-genera or even genera. But

  as the original species (I) differed largely from (A), standing nearly at

  the extreme points of the original genus, the six descendants from (I)

  will, owing to inheritance, differ considerably from the eight descendants

  from (A); the two groups, moreover, are supposed to have gone on diverging

  in different directions. The intermediate species, also (and this is a

  very important consideration), which connected the original species (A) and

  (I), have all become, excepting (F), extinct, and have left no descendants.

  Hence the six new species descended from (I), and the eight descended from

  (A), will have to be ranked as very distinct genera, or even as distinct

  sub-families.

  Thus it is, as I believe, that two or more genera are produced by descent,

  with modification, from two or more species of the same genus. And the two

  or more parent-species are supposed to have descended from some one species

  of an earlier genus. In our diagram, this is indicated by the broken

  lines, beneath the capital letters, converging in sub-branches downwards

  towards a single point; this point representing a single species, the

  supposed single parent of our several new sub-genera and genera.

  It is worth while to reflect for a moment on the character of the new

  species F14, which is supposed not to have diverged much in character, but

  to have retained the form
of (F), either unaltered or altered only in a

  slight degree. In this case, its affinities to the other fourteen new

  species will be of a curious and circuitous nature. Having descended from

  a form which stood between the two parent-species (A) and (I), now supposed

  to be extinct and unknown, it will be in some degree intermediate in

  character between the two groups descended from these species. But as

  these two groups have gone on diverging in character from the type of their

  parents, the new species (F14) will not be directly intermediate between

  them, but rather between types of the two groups; and every naturalist will

  be able to bring some such case before his mind.

  In the diagram, each horizontal line has hitherto been supposed to

  represent a thousand generations, but each may represent a million or

  hundred million generations, and likewise a section of the successive

  strata of the earth's crust including extinct remains. We shall, when we

  come to our chapter on Geology, have to refer again to this subject, and I

  think we shall then see that the diagram throws light on the affinities of

  extinct beings, which, though generally belonging to the same orders, or

  families, or genera, with those now living, yet are often, in some degree,

  intermediate in character between existing groups; and we can understand

  this fact, for the extinct species lived at very ancient epochs when the

  branching lines of descent had diverged less.

  I see no reason to limit the process of modification, as now explained, to

  the formation of genera alone. If, in our diagram, we suppose the amount

  of change represented by each successive group of diverging dotted lines to

  be very great, the forms marked a14 to p14, those marked b14 and f14, and

  those marked o14 to m14, will form three very distinct genera. We shall

  also have two very distinct genera descended from (I) and as these latter

  two genera, both from continued divergence of character and from

  inheritance from a different parent, will differ widely from the three

  genera descended from (A), the two little groups of genera will form two

  distinct families, or even orders, according to the amount of divergent

  modification supposed to be represented in the diagram. And the two new

  families, or orders, will have descended from two species of the original

  genus; and these two species are supposed to have descended from one

  species of a still more ancient and unknown genus.

  We have seen that in each country it is the species of the larger genera

  which oftenest present varieties or incipient species. This, indeed, might

  have been expected; for as natural selection acts through one form having

  some advantage over other forms in the struggle for existence, it will

  chiefly act on those which already have some advantage; and the largeness

  of any group shows that its species have inherited from a common ancestor

  some advantage in common. Hence, the struggle for the production of new

  and modified descendants, will mainly lie between the larger groups, which

  are all trying to increase in number. One large group will slowly conquer

  another large group, reduce its numbers, and thus lessen its chance of

  further variation and improvement. Within the same large group, the later

  and more highly perfected sub-groups, from branching out and seizing on

  many new places in the polity of Nature, will constantly tend to supplant

  and destroy the earlier and less improved sub-groups. Small and broken

  groups and sub-groups will finally tend to disappear. Looking to the

  future, we can predict that the groups of organic beings which are now

  large and triumphant, and which are least broken up, that is, which as yet

  have suffered least extinction, will for a long period continue to

  increase. But which groups will ultimately prevail, no man can predict;

 

‹ Prev