Each higher type is reached by adding to a lower. The higher thus embraces all that can be found in the lower, and something besides. This method is invariable, and can never be departed from. The genus must always be predicable of every individual component of every species contained under it. Translating this law into the forms of material expression, and it requires each higher species to physically include all lower species, and to differ from them only by addition. Man, the highest type, must thus include all the attributes of the cell as physically expressed, and without them he would not be man. The differences between no two terms in a series can be total. If the successive steps in a train of thought must be related, so that no two notions will be wholly distinct from each other, these notions will constitute a series, each term of which will, in a measure, determine the next, so soon as the law of the series is discovered; and if this train of thought be objectively presented, it will afford a corresponding series of physical terms, each one of which will in like manner determine the next. But thought is impossible unless by a train of ideas so related. Its physical expression will therefore be equally impossible except by a series of physical terms similarly related, each one of which in some manner determines the next. There must then be a perfect continuity in the line that reaches from the simplest form of matter through all grades of organic life up to man, the highest expression of the divine idea. There can be no break in the chain of thought, because the law of the logical process forbids it: there can be no break in the series of material symbols for the conditions of concrete expression equally forbid it. A symbol is nothing except as it represents that which is to be symbolized. So the symbols form a physical series, because the thoughts symbolized form a logical series. If the creator has fully revealed his thought, it must be by a series of physical terms arranged in such a manner as to indicate the logical series of ideas symbolized. Every form of matter is a symbol of thought, and challenges interpretation. Every change in form corresponds to an antecedent change in idea, and must be intended to reveal it. As thought, then, begins its evolution with the general and proceeds to the individual by a series of terms each of which is similarly related to both extremes, we must find the material enunciation of this process assuming the form of a series of terms, beginning with mere nebulous matter, grading into organic life, and organic life presenting us with a similar series beginning with the mere cell and ending with man. So rigid and invariable must this serial arrangement be that if a term in either series be wanting, we are authorized to hypothetically interpolate it.