Some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre existing forms. This view has been ably maintained by many authors. Until recently the great majority of naturalists believed that species were immutable productions, and had been separately created.

“I will here give a brief sketch of the progress of opinion on the Origin of Species. Our oldest cultivated plants, such as wheat, still yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification.” No case is on record of a variable organism ceasing to vary under cultivation. 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. There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food. 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. “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.

The origin of species by charles darwin