Thomas Henry Huxley quote: “There is but one hypothesis regarding the origin of species of animals in general which has any scientific existence

“Mr. Darwin and Mr. Wallace say that the opposite hypothesis, which assumes that every species is capable of varying indefinitely from its original type, is not a whit more arbitrary, and has this manifest claim to be preferred, that it will account for a multitude of phenomena which the ordinary theory is incapable of explaining.”

Charles Lyell (1797–1875) British lawyer and geologist

Source: The Geological Evidences of the Antiquity of Man (1863), Ch.21, p. 411
Context: Every naturalist admits that there is a general tendency in animals and plants to vary; but it is usually taken for granted, though we have no means of proving the assumption to be true, that there are certain limits beyond which each species cannot pass under any circumstances, or in any number of generations. Mr. Darwin and Mr. Wallace say that the opposite hypothesis, which assumes that every species is capable of varying indefinitely from its original type, is not a whit more arbitrary, and has this manifest claim to be preferred, that it will account for a multitude of phenomena which the ordinary theory is incapable of explaining.

“It cannot be doubted, I think, that Mr. Darwin has satisfactorily proved that what he terms selection, or selective modification, must occur, and does occur, in nature; and he has also proved to superfluity that such selection is competent to produce forms as distinct, structurally, as some genera even are. If the animated world presented us with none but structural differences, I should have no hesitation in saying that Mr. Darwin has demonstrated the existence of a true physical cause, amply competent to account for the origin of living species, and of man among the rest.”

Thomas Henry Huxley (1825–1895) English biologist and comparative anatomist

Source: 1860s, Evidence as to Man's Place in Nature (1863), Ch.2, p. 126

“It appears to us to be one of the many peculiar merits of that [Mr. Darwin's] hypothesis that it involves no belief in a necessary and continual progress of organisms.”

Thomas Henry Huxley (1825–1895) English biologist and comparative anatomist

1860s, Criticisms on "The Origin of the Species" (1864)

“The supporters of the Development Hypothesis… can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions.”

Herbert Spencer (1820–1903) English philosopher, biologist, sociologist, and prominent classical liberal political theorist

They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.
The Development Hypothesis (1852)

“The supporters of the Development Hypothesis… can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones.”

Herbert Spencer (1820–1903) English philosopher, biologist, sociologist, and prominent classical liberal political theorist

The Development Hypothesis (1852)
Context: The supporters of the Development Hypothesis... can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.