charles_darwin

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Primary Documents Relating to Darwin and Darwinism
with those undergoing modification and improvement, will naturally
suffer most. And we have seen in the chapter on the Struggle
for Existence that it is the most closely-allied forms,—varieties of
the same species, and species of the same genus or of related genera,—which,
from having nearly the same structure, constitution,
and habits, generally come into the severest competition with
each other. Consequently, each new variety or species, during the
progress of its formation, will generally press hardest on its nearest
kindred, and tend to exterminate them. We see the same the
process of extermination amongst our domesticated productions,
through the selection of improved forms by man. Many curious
instances could be given showing how quickly new breeds of cattle,
sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the longhorns,
and that these ‘were swept away by the short-horns’
(I quote the words of an agricultural writer) ‘as if by some murderous
pestilence.’ 10
Source: On the Origin of Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle for Life (London: John
Murray, 1859), 105–106, 110–111.
Document 16: The Origin of Species:
The Transmission of Variations
Although Darwin was convinced that his theory was the best
explanation for the origin of species, he admitted that he could not
explain precisely every aspect of his theory. One of the questions
Darwin could not answer was why some species adapted and others
did not. What caused a species to start mutating? In several places
in The Origin of Species, Darwin concedes that ‘‘our ignorance of laws
of variation is profound.’’ 11
Nonetheless, in the fifth chapter of The Origin of Species, Darwin
postulated some reasons why variation or mutation might occur. (Typical
of the nineteenth century, Darwin called these ideas laws.) In this
excerpt, Darwin suggests that the variation of a species is connected
with reproduction: this was the closest Darwin came to working out
that the mutation of genes was the cause of transmutation. Without
knowing about the action of genes, Darwin concludes that, whatever
caused the initial mutation, the accumulation of many mutations and
modifications ultimately led to the transmutation of species.
In the twentieth century, Neo-Darwinists such as the American
zoologist Theodosius Dobzhansky incorporating the work of early