The Origin of Species: Illustrated

by Charles Darwin

It is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the GENERATIVE VARIABILITY, as it may be called, still present in a high degree.

that the more an organ normally differs in the different species of the same group, the more subject it is to anomalies in the individuals.

the points in which all the species of a genus resemble each other, and in which they differ from allied genera, are called generic characters; and these characters may be attributed to inheritance from a common progenitor, for it can rarely have happened that natural selection will have modified several distinct species, fitted to more or less widely different habits, in exactly the same manner: and as these so-called generic characters have been inherited from before the period when the several species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day.

it is probable that they should still often be in some degree variable—at least more variable than those parts of the organisation which have for a very long period remained constant.

It is a remarkable fact, that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the species of the same genus differ from each other.

to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied, to natural selection having more or less completely, according to the lapse of time, overmastered the tendency to reversion and to further variability, to sexual selection being less rigid than ordinary selection, and to variations in the same parts having been accumulated by natural and sexual selection, and thus having been adapted for secondary sexual, and for ordinary purposes.

but that in each successive generation the character in question has been lying latent, and at last, under unknown favourable conditions, is developed.

As all the species of the same genus are supposed to be descended from a common progenitor, it might be expected that they would occasionally vary in an analogous manner;

If, for instance, we did not know that the parent rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether such characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blue colour was a case of reversion from the number of the markings, which are correlated with this tint, and which would not probably have all appeared together from simple variation.

When the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels.

that there is a TENDENCY in the young of each successive generation to produce the long-lost character,

Homologous parts tend to vary in the same manner, and homologous parts tend to cohere.

In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ;

for in a district where many species of a genus are found—that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work—in that district and among these species, we now find, on an average, most varieties.

Variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species,

must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state.

But when a species with an extraordinarily developed organ has become the parent of many modified descendants—which on our view must be a very slow process, requiring a long lapse of time—in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed.

Species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert t...

when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect.

we should bear in mind that animals displaying early transitional grades of the structure will seldom have survived to the present day, for they will have been supplanted by their successors, which were gradually rendered more perfect through natural selection.

It is, however, difficult to decide and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often occurring almost simultaneously.

have felt surprise when he has met with an animal having habits and structure not in agreement. What can be plainer than that the webbed feet of ducks and geese are formed for swimming? Yet there are upland geese with webbed feet which rarely go near

habits have changed without a corresponding change of structure.

that if any one being varies ever so little, either in habits or structure, and thus gains an advantage over some other inhabitant of the same country, it will seize on the place of that inhabitant, however different that may be from its own place.

and for the chance of some gradations having been transmitted in an unaltered or little altered condition.

it is not necessary to suppose that the modifications were all simultaneous, if they were extremely slight and gradual.

the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a widely different purpose, namely respiration.

in being supplied with nerves proceeding from different sources,

For these species, from belonging to distinct families, would have differed to a certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz., the nature of the organism and that of the surrounding conditions, their variability assuredly would not have been exactly the same. Consequently natural selection would have had different materials or variations to work on, in order to arrive at the same functional result;

"Nature is prodigal in variety, but niggard in innovation."

we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.

by the law of homologous variation,

and again correlation would come into play.

Several plants habitually produce two kinds of flowers; one kind open and coloured so as to attract insects; the other closed, not coloured, destitute of nectar, and never visited by insects.

but when such organs are closely examined, essential differences in their structure can almost always be detected;

For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer,

that old canon in natural history, "Natura non facit saltum."

Phenotype,development,and evolution just like distance,speed and accerleration

unity of type is explained by unity of descent.

Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former variations and adaptations, that of Unity of Type.

We thus see that with plants many morphological changes may be attributed to the laws of growth and the inter-action of parts, independently of natural selection.

have been accumulated and augmented through natural selection. A structure which has been developed through long-continued selection, when it ceases to be of service to a species, generally becomes variable,

Thus, as I am inclined to believe, morphological differences, which we consider as important—such as the arrangement of the leaves, the divisions of the flower or of the ovarium, the position of the ovules, etc., first appeared in many cases as fluctuating variations, which sooner or later became constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals, but not through natural selection;

That the individuals of the same species often differ slightly in the relative

lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable habits

either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; while the individuals less favoured in the same respects will have been the most liable to perish.

With the different species of our domesticated animals we know that the parts vary in a different manner and degree, and that some species are much more variable than others.

that a transition of structure, with each step beneficial, is a highly complex affair; and that there is nothing strange in a transition not having occurred in any particular case.

the structure of every part of its frame must be well adapted to its conditions of life.

We thus see that the first stages of the transit of the eye from one side of the head to the other, which Mr. Mivart considers would be injurious, may be attributed to the habit,

the inherited effects of the increased use of parts, and perhaps of their disuse, will be strengthened by natural selection.