K.— BOTANY. 235 



Homoplasy. 



I have already alluded to the tempestuous meeting of 1860 in Oxford. 

 Shortly after it an undergraduate came up to Christ Church who, before 

 he was of standing to take his M.A. degree, had himself made a real contri- 

 bution to the philosophy of evolution. It was Ray Lankester, who in 

 1870 published a short paper ' On the use of the term Homology in Modern 

 Zoology, and the distinction between Homogenetic and Homoplastic 

 Agreements.' ' Its author was only twenty-three years of age, and its 

 date barely a decade after the publication of the ' Origin.' This short paper 

 went far to clear up the vague ideas surrounding the term ' homology ' 

 in the minds of early evolutionists. Lankester introduced the idea of 

 * homogeny,' substituting in a more strict sense the word ' homogen ' for 

 ' homologue.' He also suggested, to avoid confusion, the use of another 

 new term, viz. ' homoplasy.' He defined homogeny as simply the in- 

 heritance of a common part, while homoplasy depends upon the common 

 action of evoking causes or of a moulding environment upon homogeneous 

 parts, or upon parts which for other reasons offer a likeness of material to 

 begin with. 



This definition was at once adopted in the morphological study of 

 animals, but Lankester did not himself apply it at the time to the 

 morphology of plants. In point of fact the conception of homoplasy and 

 the use of this clarifying term made its way but slowly into botanical 

 literature. There is reason to believe that we are as yet only beginning 

 to recognise in the evolution of the plastic plant-body how far-reaching 

 has been the influence of homoplasy, not only upon external form, but 

 also in the internal evolution of tissues. As to external form, a wide recog- 

 nition of the results of homoplasy is now generally accepted for land- 

 living plants, and in particular in respect of the origin of foliar appendages ; 

 for instance, the leaves in Bryophytes and Vascular Plants are held as 

 homoplastic, not as homogenetic ; similarly with the leaves of Bryophytes, 

 and possibly also of Pteridophytes, ivter se. On the other hand, we may 

 find among the larger Brown Algae indications of the difierentiation of a 

 supporting organ and lateral appendages from a common branch-system, 

 that can only have been homoplastic with an origin of like parts in certain 

 Red Algae. Such conclusions, drawn from the Algae themselves as well 

 as from the Archegoniatae, have the natural effect of raising distrust of 

 wide comparisons between any seaweed and any land-plant in respect of 

 foliar differentiation. Comparisons of this nature cannot be held accept- 

 able as mere guesses, by loose reference between one class and another. 

 They would have to be based on the recognition of compact sequences 

 within reasonably close circles of affinity before they could carry con- 

 viction. 



Similarly in the morphology of the internal tissue-tracts, we are already 

 familiar with certain examples of homoplasy ; for instance, that of 

 secondary thickening. No one would now hold, with the school of 

 Brongniart, that all plants with cambial activity are akin. But it is only 

 in later years that we have come to realise the far-reaching results of 

 homoplasy in the region of primary vascular morphology. MeduUation, 



'^Ann. and Mag. of Nat. Hist., vol. vi, p. 34. 



