organs which serve to give it a physical manifestation.[3] Physical life being conditioned upon organization, whenever the organism varies, the vital force thus manifested must also vary, such variation being necessarily antecedent to its manifestation. The organism varies, because it must, in order to express the added thought. Change in organism, therefore, is not induced by simple organic action, because the organs and the force acting through them can be distinguished. Assuming that matter is the objective or formal representation of thought, there can be no change in the material expression without a corresponding change in the antecedent conception. There can, then, be physical evolution, only as there is antecedent logical evolution, and then only because of this logical evolution and not because of the operation of an innate organic force. Force, whatever may be its genesis, is only the exertion of power, not the increase of it. Exertion limits the view to the force immediately in operation. We may replace one manifestation by another, but the quantity is neither increased nor diminished by this change. Change in form implies the operation of force: and apart from such manifestation in matter, it escapes the tests of science, and passes into the purely metaphysical notion of cause. And unless the operation of force be constant, or, if different forces are blended, variable according to some determinate law, the action of which is constant and discoverable, so that the different units of force are separately measurable, the force thus irregular in its action can never be placed in any scientific category. Evolution, then, cannot proceed from any innate organic impulse, unless the force that tends to exact reproduction, and the force that induces a change be equally and separately cognizable. Change must proceed according to some law which accounts for the change, and distinguishes between the normal exertion of power and that exertion which causes a deviation. Science, to be science, must explain apparent exceptions as fully as the regular operation of forces, and that which causes the irregularity must be as distinctly cognizably by itself as the force which acts regularly. Anything less than this is not science. The discovery of Neptune was the result of the application of this principle; it was a successful attempt to discriminate the force which caused variation from the force which operated regularly.

Each species represents the operation of certain vital forces, and one cannot physically pass into another except by the increase of this force, or at least by a change in the manner of its manifestation; and this increase in amount or this change in direction must separately be accounted for. Nor does it matter, for the purposes of this discussion, as to the genesis of this added increment, further than to show that its origin must be exterior to the organism by which its presence is manifested; for vital energy acting through an organism is a unit, and cannot, even in thought, be separated into distinguishable portions. Change in the direction of vital energy indicates that the original impulse has been modified in its action by encountering another force, for nothing but force can change the direction of force. It does not fall within the range of this paper to determine the nature of this exterior force which is thus distinguishable from that acting through the vital organization, and therefore capable of separate objective representation. Metaphysically we may say that force is resolvable into will, but will being purely personal is incapable of material representation, and thus cannot enter into the determinations of physical science, which does not seek to discover the origin of force, but deals solely with its presence.

As the logician must assume his premises, and, as a logician, cannot question their truth, so the physicist must assume a force in operation, and, as a physicist, cannot examine its genesis. The physical or the metaphysical method of inquiry is valid only so long as restricted to physical or metaphysical processes: a mixture of the two methods will give results satisfactory neither to science nor to philosophy. As logic furnishes no criterion by which to test the absolute truth of propositions, but deals wholly with conclusions drawn from given premises, so science furnishes no data by which to determine the absolute genesis of force, but restricts its enquiries to the phenomena resulting from a force given. For the student of physical science cause and effect is only the transference of a given and determined force from one material form to another. If this idea is to be traced further, it must be studied outside the limits of physics. This study belongs to metaphysics.

Now, if physical science does not deal with the origin of the initial force, but assumes at the outset its presence, no more does it fall within its province to examine into the origin of the increments which give to physical forms that variety which renders science possible. Science deals with results, not antecedents; and after having determined results, it is not authorized to affirm that one species has produced another by evolution, or has produced it at all. If there are agreements between different organisms by which they are brought into relation, there are also differences by which they are discriminated, and these differences imply increments of force; and to assert that one organism has evolved another is to determine not merely the presence of this new increment, but also to determine its origin. Scientific investigation deals with phenomena which give evidence to the senses of a transference of force from one form or from one manifestation to another. Transference is not increase—an effect can be no more than the evolution of what was potentially present in the cause; it cannot add to it. The origin of the force must be investigated according to intellectual laws.

It has been argued that a Supreme Intelligence in manifesting his thought will, according to the necessary laws of rational activity, pass from the universal and general to the particular and individual, or from concepts involving few attributes to those involving these and others; and that these steps in the rational process must be represented in a corresponding physical series; and that the communication of thought is conditioned upon this physical representation. If the logical series comprises one thousand terms, each related to the preceding according to logical law the physical series must comprise one thousand terms, each physically related in such a manner as to reveal this law. As the highest generalization comprises the fewest attributes, the concrete expression of this idea will present the simplest possible physical form and the least complexity of organization, and thus will present the lowest types of life; and as the individual comprises the greatest number of attributes, its concrete expression will present the greatest complexity, and consequently the highest type of life. We have seen that the logical process begins with the general and ends with the individual; its material expression must therefore begin with the lowest orders and end with the highest. But the individual cannot be immediately derived from the general without the intervention of intermediate generalizations. No more in the concrete expression of this deduction can we pass from the lowest types to the highest without the intervention of an intermediate series. These intermediate terms are not capable of independent interpretation; they find their full explanation only in the extremes of the