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).
The foot of a horse is analogous to the foot of a man, and the shell of a tortoise to the shell of an armadillo; for the two former serve for support and locomotion, while the latter two are solid protecting envelopes to the body. So also the flying organ or wing of a bat is analogous to the flying organ or wing of a beetle.
Homology refers to essential similarity in position compared with all the other parts or organs of the body, and must be considered apart from function.
Thus, as we shall see in the next Essay a single floret of the daisy is homologous with the whole flower of the buttercup. The spathe of an arum is the homologue of any bract,[97] however insignificant in size and apparently devoid of function. The foot of a horse is homologous (as we shall see later) to the middle toe only of man, while the shell of the tortoise is in part homologous with the shell of the armadillo and in part with the ribs of the latter animal.