unchanged, or with such changes only as may be attributed to varying physical influence, accident, or the interference of man. The term species was therefore made to embrace all the individuals descended from each original stock. According to this idea, there was not only an intellectual, but a real, material connection, a blood-relationship, between all the individuals of a species. Thus it has been held "that while genera, families, orders, classes, and any other more or less comprehensive division, were artificial devices of science to facilitate our studies, species alone had a real existence in nature." It has also been commonly believed that there exists between all distinct species a natural repugnance to sexual intercourse, which was designed to prevent their intermingling, and thus to keep them apart, and preserve their specific identity. This belief is confirmed by the general law of the infertility of hybrids, the few exceptions to it being considered perversions of nature or monstrosities, and therefore entitled to but little weight.

Prof. J. D. Dana, in his "Thoughts on Species,"* has given us a more transcendental definition, endeavoring to throw light upon the subject by "reasoning from central principles to the circumferential." The germ cell which contains the individual, with all its possibilities, possesses certain inherent qualities or powers; and, when surrounded by its appropriate conditions, it develops a certain specific result; and, like the molecule of oxygen, it must correspond to a measured quota or specific law of force. Therefore "a species among living things, as well as inorganic, is based on a specific amount or condition of concentrated force defined in the act or law of creation." He thus makes the fundamental distinction between species a potential one, depending on the difference of the value or law of force for each. By the same method he establishes the permanency of species. This he finds corroborated by the provisions of nature to guard their purity, as manifested by the law of hybridity mentioned above. It is perfectly consistent with this theory of the immutability of species that there should be a certain amount of variation under the varying conditions of life. But this variation is confined within fixed limits, beyond which it cannot pass. It is also temporary, and disappears with the causes which produced it. It is necessary, therefore, in studying the history of a spe* American Journal of Science, vol. xxiv, p. 305.

cies, not only to examine it in all the stages of the development of the individual, but to determine the precise amount of its variability under the varying physical influences to which it is exposed.

Perhaps Agassiz, in his essay on classification, has given the fullest and best expression to the prevailing opinions of philosophic naturalists on this subject. His views accord so completely with the most theistic opinions in natural theology, and the generally received interpretation of the utterances of the Bible on the subject, that while they claim the assent of men of science, they must be hailed by all enlightened Christians as an important contribution toward the establishment of the complete harmony of the teachings of science and revelation. In his system he admits, to its fullest extent, the doctrine of final causes. He "looks upon an intelligent and intelligible connection between the facts of nature as a direct proof of the existence of a thinking God, as certainly as man exhibits the power of thinking when he recognizes their relations." (Cont. to Nat. Hist., vol. i, p. 11.) In attempting a system of classification of natural objects, therefore, we should endeavor to discover the plan or conception which existed in the mind of the Creator, and which has been embodied or expressed in the creation. The Author of nature is the author of the true system of classification, so that in tracing it the human mind is but translating into human language the divine thought expressed in nature in living realities. In opposition to the notion of species, stated above, he contends that species have no more real existence in nature than genera, families, orders, classes, and branches have; that they all exist only as categories of thought, founded upon separate and distinct categories of characters; that these categories of thought existed primarily in the mind of the Creator, and have been embodied in living forms. He finds among animals six categories of relationship based upon structure, and states them thus:

Branches, or types, are characterized by the plan of their struc

ture;

Classes, by the manner in which that plan is executed, as far as ways and means are concerned;

Orders, by the degrees of complication of that structure; Families, by their form, as far as determined by structure; Genera, by the details of the execution in special parts; and Species, by the relations of individuals to one another and to the

world in which they live, as well as by the proportions of their parts, their ornamentation, etc.-Cont. to Nat. Hist., vol. i, p. 170.

He elsewhere, in describing species more fully, enumerates their relations under nine distinct heads, as embracing all their characteristics, and says: "As soon as all the facts bearing upon these points are fully ascertained, there can remain no doubt respecting the natural limitation of species." (Cont. to Nat. Hist., vol. i, p. 169.) He utterly rejects "as an unfailing criterion of specific identity" the law of hybridity, or, as he calls it, "the sexual connection which so naturally brings together the individuals of the same species in the function of reproduction." In this he agrees with Darwin.

It will be seen that the distinctions enumerated above, between the divisions recognized by all naturalists, are differences of kind, not of degree. It is, therefore, impossible that by variation one class of differences should pass into another; that is, specific differences become generic, or the reverse. From this, Agassiz infers the immutability of species, considering "that all organized beings are created, that is, endowed from the beginning with all their characteristics," and that these characteristics have been transmitted unchanged, except within certain limits, to all their descendants. Not only were species supernaturally created, but their geographical distribution he considers also primordial. Instead of originating in a single locality, they have been created in the localities where they now exist, not in a single individual or pair, but in a multitude of individuals, as many, probably, as have represented the species at any period of its history. This last is also opposed to the popular idea of the community of descent among all the individuals of a species. According to him, the connection, instead of being a material, is only an intellectual or ideal one. He sums up his opinion on this point in few words: "Species, genera, etc., exist as thoughts, individuals as facts." (Am. Jour. of Sci., xxx, p. 143.)

Darwin and the transmutationists, on the other hand, consider a system of classification nothing but a convenient arrangement of natural objects into groups, differing from each other not in kind, but only in degree. His theory admits the orthodox doctrine of a community of descent for all the individ

uals of a species, and their distribution by natural agencies from a single locality. But he carries the doctrine of community of descent to a most unorthodox extent. He believes that species have not been independently created, but have descended like varieties from other species. He thus states his theory at the end of his Introduction: "I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species." (Page 13.) This is quite a moderate statement of the theory; and in reading the book we are led to believe that he extends it no further. His whole argument is really intended to establish no more than this. It is not until he has reached the Conclusion that he seems to have ventured to the full extent to which it was manifest his doctrine must carry him. Here he is led to apply the theory of descent with modification to members of the same class. Then he launches out still more boldly and says: "I believe that animals have descended from at most only four or five progenitors, and plants from an equal or less number." (Page 419.) He seems inclined to stop again at this point, but a little thought soon satisfies him that there is no resting-place here. He then makes the final plunge: "Therefore, I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from one primordial form, into which life was first breathed." (Page 419.) Here at last we find the germ out of which all the diversified forms of plants and animals have been developed by the operation of secondary causes. As to the precise nature of this

primordial form," he very prudently avoids giving us any information. Had he gone one step further, and made this form the result of the action of physical forces on inorganic matter, his development theory would have been more complete; and then, by acknowledging the omnipotence of matter, he could have dispensed with a primary or efficient cause altogether. But he has not taken these last two steps, and has therefore escaped the bottomless pit of atheism which opened just ahead of him.

Starting, then, from this originally created form, the first slight modifications would give us varieties; as these became

more distinct, species would result; as these differences became greater, we would divide species into distinct groups called genera; and as they separated still wider, into families, and so on, until we reached the greatest divergence expressed by branches or types. Thus, according to Darwin, varieties are but incipient species, species incipient genera, and so on through the whole series. Agassiz has compared his system of classifying animals to the grouping of the stars; the stem and branches of a tree better illustrate Darwin's idea.

The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into

great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species into groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which have lived during long-past geological periods, very few now have living and modi fied descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favored, and is still alive on its summit, so we occasionally see an animal like the ornithorhynchus or lepidosiren, which in some degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation, I believe it has been with the great tree of life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.-Pp. 118, 119.

It will occur to the reader at once that this is no new doctrine. Passing by the crude speculations of the ancient philosophers on this subject, we find that it has been repeatedly FOURTH SERIES, VOL. XIII.-39