To emphasize this difference, botanists call such an apparent leaf a _phyllodium_, or phyllode. After these illustrations we now repeat the definitions in different words. Analogy has reference to _general resemblance_ of form determined by _similarity of function_, however different the origins of the parts compared may be. Homology has reference to _community of origin_, however obscured to the superficial observer such common origin may be by modifications necessary to adapt to different functions. Observe, then, there are two ideas here which must be kept distinct. One is common origin, always shown by deep-lying, essential identity of structure; the other is adaptive modification for function. Organs of the most diverse origin may resemble by adaptive modification for the same function. This is analogy. Organs of the same origin may assume very different appearance by adaptive modifications for different functions. This is homology. In the latter case, which is the one that concerns us, a profound study of essential structure and structural relations to other parts, and especially extensive comparison in the taxonomic and ontogenic series, will usually detect the homology, or common origin, in spite of the obscurations produced by adaptive modifications. It is seen, also, that analogy is a superficial resemblance, easily detected by the popular eye, and therefore embodied in popular language; while homology is a deep-seated and essential resemblance, detected often only by profound study and extensive comparison. Now, one of the strongest proofs of the truth of evolution is taken from the homologies of animal structure. Common origin completely explains homology. Every other explanation is transcendental, and therefore unscientific. =Primary Divisions of the Animal Kingdom.=--Now, the animal kingdom consists of several primary divisions, called sub-kingdoms or departments. The animals in these groups differ so essentially from one another in their _plan of structure_, that it is difficult, if not impossible, to trace any structural relation between them--to imagine how the members of one could have been derived from those of another--or conceive the common stem from which they all separated. In other words, it is impossible, in the present state of knowledge, to trace homology with any certainty from one group to another. But within the limits of each primary group the homology is easy. Some naturalists--Agassiz and Cuvier--have made four or five of these primary groups. Some--Huxley--have made eight. Some make nine or ten.[21] We will not trouble ourselves to settle this question; for all agree to make _vertebrata_ and _articulata_ or _arthropoda_ two of them, and all our illustrations will be drawn from these. Other groups are too unfamiliar to the general reader to serve our purpose.