68o LIFE : OUTLINES OF GENERAL BIOLOGY 



are transformed fore-limbs. It would be pedantic to make 

 much of the backward continuation of the bat's wing beyond 

 the fore-limb boundary. 



In some plants the function of a leaf is discharged by a flattened 

 iihoot, which looks extraordinarily leaf-like, as in the case of the 

 floating discs of the Duckweed (Lemna). This is a difficult case, for 

 the internal structure of the flattened shoot is also adapted to the 

 leaf-function that has been assumed. Yet it is impossible to admit 

 homolog3^ when the mode of development Is quite different. More- 

 over, in the case of the duckweed, the leaf-like floating disc bears 

 very minute flowers on its margin, and gives off a long root on its 

 under surface. A tendril may be part of a stem or part of a leaf or 

 a transformed stipule, yet the function is the same throughout. A 

 prickle on a rose-bush is an integumentary structure, comparable 

 to a hair; the spines on the holly are moribund projections on the 

 margin of the leaf; and the thorns of the hawthorn are abortive 

 branches — often with tell-tale leaves. 



HOMOPLASTY OR CONVERGENCE.— In 1870 Lankester sug- 

 gested the desirability of distinguishing homogeny, or homology due 

 to common descent, from "homoplasty", or resemblance due to the 

 occurrence of similar adaptations in unrelated forms. He defined 

 homoplasty as "that close agreement in form which may be attained 

 in the course of evolutional changes by organs or parts in two animals 

 which have been subjected to similar moulding conditions of the 

 environment, but have no genetic community of origin to account 

 for their close similarity in form and structure". The idea was 

 subsequently extended to the form of the entire animal as well as 

 the form of parts or organs. The term "convergence" was also sug- 

 gested as a synonym for homoplast3% and Weismann defined it as 

 "corresponding adaptations to similar conditions in animal forms 

 not genealogically connected with one another". The matter deserves 

 careful consideration. Convergence is now the usual term. 



We have already noticed that structures that have no deep 

 resemblance in development or in fundamental structure may per- 

 form a similar function, and may become similarly adapted to this 

 function. Thus a stem structure (or phylloclade) in Asparagus, 

 Butcher's Broom, Duckweed, and so on, may assume leaf -function 

 and may become leaf-like not only externally, but to some extent 

 internally. Such cases lead on to homoplastic resemblance or con- 

 vergence — a resemblance between unrelated organisms or organs 

 brought about by the fact that they have become similarly adapted 

 to similar functions or to similar conditions of life. The majority of 

 fishes are spindle-like or torpedo-like in shape, no doubt in adapta- 

 tion to swift movement in the water, but there is no need for a 



