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. There is no relation of homology, however remote, between the wings of a bat and of a beetle, and these two animals (as will shortly appear) have the parts and organs of their bodies so fundamentally different, that it is doubtful whether any definite relations of homology can be established between them. A special term has been devoted to signify a resemblance between two parts in two different animals and plants, which resemblance has been induced by or is directly related to their common needs, and the similarity of external influences. This term is "homoplasy," and structures which may thus be supposed to have grown alike in obedience to the influence of similar external causes acting on similar innate powers have been called _Homoplasts_. Such, then, are the more general conditions as to structure and figure which living creatures present, and (as has been said) with great differences as to the amount of possible variation, most kinds have a definite limit as to size. It remains only to make general observations on the colours of living creatures. But a few years ago, hardly any few general remarks of really scientific interest and value could have been made respecting the varied hues and markings which organisms present. No rational relation was even suspected to exist between the colours of plants and the busy insect life which swarms about their blossoms or about the varied colours of birds, and the details of their habits and modes of existence.