scientific evidence nothing involving conflict with religious conviction; and allowance will be made for continued and extended experiments, provided only that the "holding open the possibility," does not make the probability "self-evident."

Passing thus from the unsolved problem of the origin of life,—with the attendant acknowledgment that there is no scientific theory of the world's descent from primordial atoms, we advance to the testimony of science concerning different orders of life, and their relations to each other. Here there is no restriction as to the use of scientific methods; there is wide range for free action by the hosts of observers required for the work. A sense of the relief which this implies, in contrast with the hampering restraints surrounding the earlier question, imparts to the words of Schmidt a more jubilant tone, as he says, "Between beginning and end, we naturalists may do as we please."*

In this region it will be universally admitted that the development of species by selection is the conception which has gained greatest hold upon the scientific mind within * Doctrine of Descent, p. 162.

recent times. Accordingly it seems better to begin with an outline of this theory, and of its history as developed by Mr. Darwin, keeping, however, steadily in view the range of problem with which the theory has to deal.

As already remarked the origin of life is a problem quite distinct, and the theory of selection does not entangle itself by maintaining origin of life by development from unorganized matter. Mr. Darwin clearly guarded himself against responsibility for such a conception of its origin.* Restricting thus far, we have to keep in view the vast range of the problem demanding scientific explanation.

Animal life manifests itself by spontaneous movement,-movement from within the living organism, in contrast with movement caused by energy applied from without. This holds true of every living germ, from which a mature animal form may be developed. This is the test of the presence of life under the microscope in minute germinal structures otherwise incapable of observation. In its lowest known form life appears in a minute vessel containing so much nourishment, which is within a nucleus or vital centre from which move

* Origin of Species, p. 577.

ment originates.* This depends for its development on external conditions such as heat and moisture. Advance in the scale is connected with greater complexity in the organism. As we ascend, separate organs appear, fulfilling distinct functions, and controlled by means of a nerve system whose ramifications are according to the intricacy of the organism. The appearance of such a nerve system implies sensibility and motor activity, provision for an experience more or less varied, and movement in some measure according with it, or adapted to it. From a nerve system consisting of a centre and a few fibres, we gradually ascend, until in the higher vertebrate orders, we find a vast system of nerves harmonized and governed from a grand centre or terminus constituting the brain. With the steadily advancing complexity of nerve system, there is great diversity in the orders of animals, in the variety of actions of which they are capable, in adaptation to different conditions of life, and in the modes in which they obtain subsistence. The scientific problem is-How far can we account for all this complex system of things by the action of natural causes? As we are here specially concerned with * See Appendix VII. † See Appendix V. ‡ See Appendix IV.

ruling conceptions, and only indirectly with details, the relations of the animal and vegetable kingdoms may be passed, with the acknowledgment that the two are obviously distinct, and yet that the vegetable very nearly merges into the animal at certain points, as illustrated by sensibility, if not by locomotion. So may we waive discussion of the classification of animals, which involves important points closely connected with the main question. Allowing that the wonderful diversity in animal organization, may admit of different modes of classification; and yet granting that the structure of the animal economy in all its forms proceeds on common principles, which must lead to practical agreement as to modes of classification; we deal with the single question as to the natural causes serving to explain different orders.

Mr. Darwin's suggestion, largely supported by evidence from many quarters, and now commanding a very general support is, that all this diversity may be regarded as the product of a long process of development. This theory takes the whole orders of existence into one sweep. The lowest type of

animal life presents to view the beginning of a protracted history; the highest order of animate existence indicates the stage of advance at which we observe things at the present period. For such a theory the history of individual development belonging to whatever order, and the history of distinct orders as indicating improvement and deviation of whatever aspect, present evidence of special value. The most important causes relied upon as contributing to the formation. of a scientific theory may be presented under these four heads-(1) The action of external causes as provided for by environment; (2) power of adaptability within each organism, providing for changes according to requirement from without, which may be described as "adaptive changes of structure;" (3) "the struggle for existence," in accordance with which the strongest gain the mastery and consequent advantage in obtaining the means of subsistence; and (4) "natural selection" among the sexes, giving to the offspring all advantages according to the laws of hereditary descent.

The essential feature in this theory is the power of adaptability inherent in the organism.