It was during this voyage that Darwin first began to think of the origin of the different kinds of animals, and to wonder how far circumstances and special conditions went in changing one species to another almost entirely different.

And this question, which it was the aim of his life to settle, he studied patiently for the next twenty-one years. In the grounds around his house and in the conservatories, aquariums, and rooms for collections he studied the phenomena of plant and animal life from the stages to the latest; comparing his conclusions with the views of all the great modern scientists, whose works found ready welcome to his library.

These studies were so exact and thorough that Darwin gained a reputation for accuracy which was of great service to him when he brought out the great theory which was destined to meet with such bitter opposition.

As the result of the years of labor he published in 1857 his great work on the "Origin of Species," which has done more to change the current of scientific thought than any other work of the century.

Although the great central thought of the book was not original with Darwin, it was to his untiring efforts and exhaustive studies that its acceptance by the scientific world was due. His wide experience and years of careful investigation gave his words a special value, while the generalization, or summing-up of the scattered facts, developed the hint of the older naturalists into the almost impregnable theory which Darwin sought to make it.

In "The Origin of Species" all the different varieties of animals are accounted for as due to changes in circumstance, on the theory that any organ of an animal which is not used will gradually become useless, and one that is much used, or put to uses for which it was not at first intended, will grow larger and stronger, and change its appearance to suit its new work.

Thus the fishes in the Mammoth Cave are blind because the darkness there has made the eyes useless for many preceding generations; on the contrary, the eyes of the eagle, whose mode of life necessitates great strength and clearness of vision, would grow stronger and stronger with each age; and if, by some accident, the fish or bird were transferred to an entirely new element, the organs of their descendants would be so modified to suit their new life that they would be entirely changed in appearance.

It must be remembered that these changes occur at great intervals of time, and that though any observing boy would be able at once to detect the difference between a wild carrier pigeon, these differences are only the result of conditions which have existed for a comparatively short time, while ages would elapse before the winged reptile would develop into a bird.

This theory of the development of species, which will always be connected with Darwin’s name, was also advanced by Alfred Wallace, a Welshman, who was born in 1824, and who arrived at the same conclusion as Darwin, although each naturalist was ignorant of the other’s views until they were matured. In 1858, before the publication of Darwin’s book, Wallace sent home from the Malay Archipelago a pamphlet containing a theory of the origin of species which was practically the same as Darwin’s, and the ideas of both naturalists were made known to the scientists of London at a meeting of the Linnæan Society in the same year.

Thus to both belong equal honors, and that two men, working quite independently, should arrive at the same result, is another instance of the fact that the realm of science belongs to no nation or age or individual, but is the common heritage of all.

The publication of the "Origin of Species" produced an immense sensation, and resulted in the division of scientists into many sects, who arrayed themselves as bitterly against one another as did the disciples of Aristotle against the followers of Copernicus; and we must recall the persecution of Galileo by the Inquisition to find a parallel for the vindictive enmity which followed Darwin for the next few years.

But what the telescope did for the Copernican system, geology has done for Darwinism, and in the fossils of the far-off ages we read the same story that is written in the pages of Darwin.

Two years after the publication of the "Origin of Species," a fossil was found of a creature with the wings, feathers, feet, and breast of a bird, and the head, teeth, and the tail of a lizard; an unmistakable proof of the former existence of a class of animals between reptiles an birds. And further study of the fossils has revealed other intermediate forms of life just as remarkable. Fossils of reptiles standing on hind legs like those of the kangaroo, and fossils of birds with teeth have been found, while the forms of extinct quadrupeds show that the horse, with its hoofed foot, is descended from a much smaller animal with five toes, and that cats, dogs, bears, and many other animals differ as much-to-day from their remote ancestry as the butterfly differs from the caterpillar. And in the vegetable world the same thing has been found true, and the plants of to-day are connected in the same mysterious way with those of past ages, the form of fern and lily being but a repetition of the forest trees, and gigantic blossoms of the older world.

And while geology has been the guiding light illuminating the past, so that its records might be thus easily read, the living world, too, adds its proofs that the changes alluded to by Darwin are still going on, and that in this as in the other things the past may be studied from the present.

The animals that inhabit islands, of the same species as those of the mainland, have their habits and organs altered by their changed conditions; the birds, lizards, and insects of the desert are all of the neutral tints which correspond with the prevailing color of their surroundings—because those are the conditions which best protect them against their enemies, who would be more easily attracted to them if their color were brilliant. . The fur of the same species of animal is thicker in the north than in the south, and the same kind of shells differ in depth of color as they are found in deep or shallow water. And an infinite number of other instances might be shown to prove that the same forces which changed the scale-covered reptile to the bird furnished with wings and feathers are still at work.

And thus, just as Newton and Herschel connected the earth by a magnetic chain to every star of heaven, so Darwin joined all the visible forms of life, and proved that each plant and animal is a link in the same chain, bound together by a power as subtle as that which holds the stars.

Among the great thinkers who have helped to popularize the opinions of Darwin, Professor Huxley, born in 1825, must rank first. Huxley's investigations have followed the same lines as those which mark the labors of Darwin, and his independent researches and splendid work for science have done much to place the Darwinian theory on a firm basis. This is due, first, to the fact that Huxley's original work is of such merit as to make his opinions carry great weight, and, secondly, because he has the gift of the interpreter, and whether he speaks of his own communings with nature, or translates the words of another, he is equally powerful and convincing; and the work of Darwin, which to many might have remained a sealed book, has been by the genius of Huxley rendered comprehensible, just as the works of a great composer reach the multitude througlh the medium of the performer, and the written notes become exquisite melody.

Huxley's own works comprise studies in almost every department of zoology, and are remarkable for their originality and depth of thought, and will ever be considered as independent and valuable supports of the new school of thought.

The studies of the forms of animal life have led to a truer knowledge of the laws of development than the older naturalists thought it possible to attain, and the mystery of life has been invested with a new interest from the discovery of the close connection between the animal and vegetable worlds.

In his careful studies of organic life the naturalist has found it difficult often to tell where the one world leaves off and the other begins. Plants have been found so closely resembling animals as to make their place in the world of nature doubtful, as for instance the Sundew, which, by a singular arrangement of its organs, is able to capture insects and digest them by a process like that of animal digestion, and to feel the effect of anesthetics. On the other hand, certain organisms, as the sponge, which have been placed in the animal kingdom, are plant-like in their habits.

Chemistry, electricity and the microscope have been effective agents in the study of the development of the different forms of life, and the results have been such as to place biology among the leading sciences of the day.

Whether future study will reveal secrets that elude the biologist of to-day and discover yet closer relations between all forms of life is a question which carries its own answer with it, as the history of science has shown that the questions which one age asks the succeeding age answers, and that the progress of scientific thought is, on the whole, as sure as the growth of the oak from the acorn.

The science of the present day has made its most far-reaching generalization in the statement that no energy is ever lost, but only changes form; the muscular force of man may, by the rubbing together of two pieces of wood, produce heat, and the heat light, and light is absorbed and transformed again into heat, or converted into chemical energy. Thus everywhere is seen one form of energy changing to another, and gradually tending toward heat.

As the life of man is bound up with and dependent upon the mysterious forces of nature, his interest in them can never cease, and thus the last word for science can never be written while the race endures.