Naturæ, contains the names of the species new to science added to our lists during the year of which it treats; and in the record of each year we find the names of two or three times as many as are mentioned in the whole Systema Naturæ. Yet the field shows no signs of exhaustion. As these volumes stand on the shelf together, it is easy to see that the later volumes are the thickest, and that the record for the present year is the largest of all. The additional species named and described in 1889 are more than ten thousand. Moreover, what is true of the increase of knowledge in systematic zoölogy, is even more marked in the case of botany. Such, then, is the variety of life on the globe—a variety of which Linnæus and his successors had never dared to dream.
And yet, great as this variety is, there are, after all, only a few types of structure among all animals and plants—some three or four or eight or ten general modes of development—all the rest being minor variations from these few types.
It is even true that all life is but a series of modifications of a single plan; for all organisms are composed of cells, the essential element of which is always a single substance—protoplasm. All are governed by the same laws of development, reproduction, and susceptibility to outside influences. Unity in life is therefore not less a fact than is life's great diversity. In whatever way we account for the diversity, the essential unity must not be forgotten. The bonds of unity among organisms constitute what the naturalist calls homology.
That these resemblances have some deep significance, no thoughtful student of nature has ever doubted. What this significance may be is the underlying question in that branch of philosophy which has come to be known as evolution.
In the present discussion I shall take for-granted that answer to these questions which is associated with the name of Darwin; and, as a student of the relations and distribution of animals, I firmly believe that no answer to these questions fundamentally different from his will ever be possible.
The essence of the Darwinian theory is this, that the various species of the present day are all derived from pre-existing forms, more or less unlike them; that this derivation takes place through the operation of natural laws—the law of heredity, the law of response to external stimulus or environment, and the law less clearly understood by which variations from ancestral types are constantly produced; the "divine initiative" in the individual which struggles against sameness and monotony. The constant tendency of organisms to multiplication by geometric progression in a world of limited extent, already apparently full, brings about a constant struggle for existence among these organisms, and by this struggle, we have the progressive adjustment of individuals
