EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 345 



Examples of analogy of parts have already been noted : for instance, 

 in those woody climbers which support themselves by reflexed hooks 

 (Fig. 143). These hooks may actually be axillary buds (Sageretia, 

 Carissa), or reflexed pinnae (Desmoncus), or recurved stipules 

 (Zizyphus), or merely superficial prickles (Lantana, Calamus). Func- 

 tional similarity in parts of diverse origin is seen also in tendrils, 

 which may actually be axes (Vitis, Bauhinia), or leaf-blades (Lathyrus 

 aphaca, Fig. 148), or distal parts of the leaf (Cobaea, Vicia cracca), 

 or lateral appendages at the base of the leaf (Smilax). Again, spinous 

 development may appear in stems (Crataegus), or leaves (Berberis), 

 or stipules (Acacia), or in roots (Ac author hizd) . On the other hand, 

 axillary buds may develop flattened and leaf-like, as in the " phyllo- 

 clades " of Ruscus (Fig. 259). But their position, and the fact that 

 they bear flowers proclaim their shoot-character, notwithstanding 

 their leaf-like appearance. Such examples might be indefinitely 

 multiplied, showing how common is homoplasy in parts that are 

 only analogous, and not really comparable one with another in point 

 of origin and position. 



There may be various degrees of that closer correspondence of 

 parts which would justify their being held as homologous. The 

 strictest conception of homology is that designated Homogeny. Lan- 

 kester defined as homogenous those structures which are genetically 

 related in so far as they have a single representative in a common 

 ancestor. This definition implies repetition of an individual part 

 bearing a definite relation to the whole organism, just as the hand of 

 a child repeats in position and in its qualities the hand of the mother. 

 Clearly, in Plants with their continued embryology, the recognition 

 of such individual correspondence is rarely possible. The indefinite- 

 ness in number of the appendages produced by any growing shoot 

 precludes it. And yet the leaves of a shoot are comparable in other 

 respects. Thus it is only in a less stringent sense than homogeny that 

 the homology of the foliar appendages can be admitted in organisms 

 which produce an acropetal succession of them as flowering plants do. 



It may be suggested that the successive production of appendages 

 on the growing axis of a plant finds its correlative in the " serial 

 homology," or, as it has been called, the " homodynamy " seen in 

 animals. But the " segments " of the animal body have not their 

 counterpart in the plant-body. Attempts have been made, it is 

 true, to reduce the shoot to an articulate series of constituent parts, 

 each consisting of a portion of the axis bearing one or more appen- 

 dages. But though it is possible to analyse some few shoots in this 



