Darwin's terse statement of his conception was: "As many more individuals of each species are born than can possibly survive, and as consequently there is a frequently recurring struggle for existence, it follows that any being, if it vary in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance any selected variety will tend to propagate its new and modified form." ("Origin of Species," Intro.) This statement of the doctrine of the survival of the fittest, suggesting a glimpse at the great pageant of nature from the remotest times, shows how the organisms existing at this moment are the descendants of the victors in the world's greatest battles. The struggles for life, always keen and persistent, shared in by every individual organism, both animal and vegetable, are the instigators of all progress in the natural world. They are nature's means for the attainment of beauty, usefulness, and perfection. The Darwinian theory was based upon the observed facts that members of any given species are not alike, while their offspring may differ in numerous ways from their parents. The data furnished by zoölogy, botany, physiology, and other sciences supply overwhelming evidence that the present species of animals and plants have arisen through the modification from various causes of many pre-existing species. The organisms with which we are familiar owe their characteristics to the accumulation of a long series of changes similar to those that we may see that they are still undergoing. 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.