Thus, the overwhelming majority of living creatures are, as has been said, formed of aggregation of cells, which cohere or fuse together in various ways--and not only of aggregation of cells but of aggregation of aggregations of cells or "tissues." Each tissue is a structure formed by the aggregation, or by aggregation and metamorphoses, of certain sets of cells. Thus, every higher plant or animal is made of an inconceivable multitude of cells, together with tissues which are not cellular, but which have originated by metamorphosis of cells, and every such higher plant or animal at first consists entirely of an aggregate of plainly distinct cells; and, first of all, of one single cell only, whence its whole structure, however complex, has originally sprung, though generally not until it has had at least a portion of another cell mixed with it. This transformation of cells, at first all alike, into distinct orders of cells or _tissues_, whence different organs with different functions arise, is characteristic of all living creatures above those which each consist throughout life of one cell only. We have seen that unicellular organisms may unite into a cylindrical or spheroidal colony, as in some _Radiolaria_, or into a spheroid of closely-adjusted cells, forming one layer, as in _Volvox_. But however large or complex such aggregation may be, it never forms sets of united cells or tissues. The whole of these lower creatures, therefore, may be spoken of as unicellular organisms; as though they may consist of many cells, those cells retain their individuality. Such creatures are all the lowest animals--those called _Hypozoa_[94] or _Protozoa_, and also the lowest cryptogamic[95] plants. All other animals and all the higher plants are multicellular. The description of one animal (which is placed as it were on the boundary between the multicellular and the unicellular division), the little parasitic worm _Dicyema_,[96] must for the present be postponed, as its significance could not yet be understood. Before leaving the consideration of the forms of living creatures, a further distinction should be made clear--that is to say, a distinction in the nature of resemblances which may exist between various parts. There are two different relations which may exist between a part or organ in one animal or plant, and another part or organ in another animal or plant. One of these relations is called _analogy_ and the other _homology_, and it is very desirable to bear clearly in mind the distinction which exists between these two relations. _Analogy_ refers to the use to which any part or organ is put--that is, it refers to its function. Thus, the flower of the daisy is, as we shall see, analogous to that of the buttercup. The spathe of an arum is analogous to the corolla of the dead nettle (for both serve to shelter the essential parts of the flower).