On the Origin of Species HTML version

Variation Under Domestication
Causes of Variability -- Effects of Habit and the use and disuse of Parts -- Correlated
Variation -- Inheritance -- Character of Domestic Varieties -- Difficulty of distinguishing
between Varieties and Species -- Origin of Domestic Varieties from one or more Species
-- Domestic Pigeons, their Differences and Origin -- Principles of Selection, anciently
followed, their Effects -- Methodical and Unconscious Selection -- Unknown Origin of
our Domestic Productions -- Circumstances favourable to Man's power of Selection.
When we compare the individuals of the same variety or sub-variety of our older
cultivated plants and animals, one of the first points which strikes us is, that they
generally differ more from each other than do the individuals of any one species or
variety in a state of nature. And if we reflect on the vast diversity of the plants and
animals which have been cultivated, and which have varied during all ages under the
most different climates and treatment, we are driven to conclude that this great variability
is due to our domestic productions having been raised under conditions of life not so
uniform as, and somewhat different from, those to which the parent species had been
exposed under nature. There is, also, some probability in the view propounded by
Andrew Knight, that this variability may be partly connected with excess of food. It
seems clear that organic beings must be exposed during several generations to new
conditions to cause any great amount of variation; and that, when the organisation has
once begun to vary, it generally continues varying for many generations. No case is on
record of a variable organism ceasing to vary under cultivation. Our oldest cultivated
plants, such as wheat, still yield new varieties: our oldest domesticated animals are still
capable of rapid improvement or modification.
As far as I am able to judge, after long attending to the subject, the conditions of life
appear to act in two ways--directly on the whole organisation or on certain parts alone
and in directly by affecting the reproductive system. With respect to the direct action, we
must bear in mind that in every case, as Professor Weismann has lately insisted, and as I
have incidently shown in my work on "Variation under Domestication," there are two
factors: namely, the nature of the organism and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations sometimes arise
under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar
variations arise under conditions which appear to be nearly uniform. The effects on the
offspring are either definite or in definite. They may be considered as definite when all or
nearly all the offspring of individuals exposed to certain conditions during several
generations are modified in the same manner. It is extremely difficult to come to any
conclusion in regard to the extent of the changes which have been thus definitely
induced. There can, however, be little doubt about many slight changes, such as size from
the amount of food, colour from the nature of the food, thickness of the skin and hair
from climate, etc. Each of the endless variations which we see in the plumage of our
fowls must have had some efficient cause; and if the same cause were to act uniformly