The methods pursued in studying variation in species, and its important accompaniment, heredity, consists in comparison, statistical examinations, cultural experiments, and crossbreeding.
Evolution is the process of differentiation accompanying the operations of nature. All the great naturalists before Darwin's time noted facts indicating this universal differentiation, but it required the particularly wide sweep of Darwin's mind to phrase and demonstrate it.
The law of origin by evolution, as Herbert Spencer showed, is not confined to the method of bringing into existence new species of animals and plants. The stars, planets, the geological strata and earth contours and forms, human institutions, social customs, and practically everything in nature are obedient to it.
Much research work in evolution has been done since Darwin stated his theory, but the basic principle of the survival of the fittest remains untouched by criticism. Some of his views respecting minor details of selection and the effects of various factors have been modified or enlarged, and many new evolutionary forces have been discovered. It has also been found that a single cause is usually followed by more than one effect.
Weissmann has drawn attention to the importance of adaptations. Most organic beings are usually closely fitted for the conditions under which their lives are spent.
The principal parts of every animal and plant, and all the points in which one species differs from a nearly related species, have been shown to have arisen on account of their usefulness to the creatures possessing them. As natural selection is always progressive, it follows that no adaptation is ever perfect. There is always progress from the useful to the more useful—a continual striving for greater beauty of form and color and higher efficiency.
Works on evolution furnish an abundance of interesting evidence showing how adaptation works. A single instance may be cited here.
One of the Mexican yucca plants common in our Southern States is pollinated by a moth of the Pronuba family. This moth is adapted for its work by several special organs including a special ovipositor and peculiar maxillary tentacles which are not found in other moths. The female moth collects pollen with these tentacles from several yucca flowers, rolls it into a ball and kneads it into a pellet. When the pellet is ready the moth seeks an unvisited flower and, after depositing a few of her own eggs in the ovary, she climbs the style and forces the pollen pellet into the stigma. This is the way the yucca is pollinated and fertilized. Two important purposes are served by this arrangement: a species of plant and a species of moth, together with those dependent upon them, are enabled to survive by this moth's activities. There are many known cases of similar cooperative adaptation to living conditions.
Quetelet, in 1845, followed by Francis Goltin and Karl Pearson, have applied statistical methods in dealing with evolutionary problems, and a new science called biometry has been developed. This science has yielded much important data regarding the effects of inherited characteristics.