The differences between the terms in this series must be cognizable. Two terms which are indistinguishable are practically identical; and two terms which are not identical vary by a difference which is cognizable by itself apart from either term. The steps in the logical evolution of the final term. _Being_ must be separable to be cognizable, and the material forms interpreting these steps to the senses must also be distinguishable. A species differs from the genus by the addition of at least one attribute. Now, if the species is distinguishable from the genus, the attribute which differentiates it, must be separately cognizable--so also the individual differs from the species by the addition of attributes, which must in like manner be separately cognizable, or the species will never be conceived independently of the individuals. A thought cannot proceed by insensible steps, nor can its material expression vary otherwise than by determinate and distinguishable differences. The distinction of species is thus a logical necessity. The addition of distinct attributes to the genus gives origin to distinct species; variation in attributes not affecting their substantial identity gives rise to varieties. One species, then, cannot become another, except by the assumption of a new specific attribute, so that one species passes into another precisely as the genus passes into the species, and that is just as, and not otherwise, than one thought passes into another. The fundamental law of the logical process is that we pass from the generic towards the individual; from the simple to the complex. Induction can proceed only by assuming a genus at the outset--that is, by assuming certain attributes in the individual to be generic. Translate this law into material forms, and we have each higher--that is more complex--species evolved from the lower by the addition of some new characteristic. This new attribute cannot be added by the functional activity of the lower organism; that can only reproduce itself. A thought does not change merely through repeated expression. We pass to the conclusion of a syllogism, not from each term, but from a comparison of the premises--and this requires an intellectual operation entirely distinct from a mere apprehension of the terms. It is one thing to comprehend the premises; it is quite another to deduce a conclusion from them. It may necessarily follow, but it requires a separate act of the mind to reach it. Premises will not of themselves reach a conclusion.