1028 REPORT— 1898. 



Starting from this repetition of individual parts in plants nearly related, there is 

 a divergence by gradual steps in two directions: Firstly, in the individual plant, 

 ■where the later formed parts may assume forms and positions which may even raise 

 a question of their essential correspondence. Thus in the batch of Sunflower 

 seedlings there may be a varying number of leaves, with varying transition from 

 the decussate to the alternate arrangement, intervening between the cotyledons 

 and the capitulum. As they vary in number and position these cannot in the 

 strictest sense be accepted as individually comparable, each to each by descent — the 

 lineal representatives of like individual parts in the parent. The lateral roots also, 

 though all essentially similar, do not correspond each to each, either in number 

 or in position. 



Again, to go a step further, a Fern prothallus produces antheridia and arche- 

 gonia ; their number and position are not uniform ; oy conditions of culture we have 

 them under control, and can induce antheridia only, or we can induce a formation 

 of archegonia upon the upper surface, where they are usually absent. Plainly 

 these cannot be held severally as the exact representatives of like individual parts 

 in a previous generation. Another exceptional but most interesting case is that 

 of Aspidium anomalum, Hk. and Arn., which Sir William Hooker remarks is 

 possibly an abnormal form of Aspidium (Polyst.) aculeatum, Sw. In this Fern the 

 sori, instead of being all on the lower surface, as in allied Ferns, are often upon the 

 upper surface of the leaf. There is no sign of tortion to explain the anomaly, 

 while the sori themselves present no structural peculiarity except that they are 

 sometimes quite destitute of indusium. There has doubtless been a transfer of 

 developmental capability from the usual position of the sori to the anomalous one. 

 In case of such transfers as these we do not doubt that the parts in question are 

 to be ranked as comparable to those in the normal position ; we contemplate here, 

 as in the case of the Sunflower leaves, an essential correspondence, but not an 

 individual repetition of the parts, and we learn that parts thus essentially corre- 

 sponding to one another may be transferred to unusual positions. 



Secondly, in plants more or less nearly related, those which are less akin may 

 show so slight a similarity in detail that again questions of the essential correspon- 

 dence of the parts may arise. Within nearer circles of affinity these questions will 

 affect only the appendages of minor importance, which show less constancy of occur- 

 rence and arrangement, such as emergences and hairs ; but in case of plants less 

 nearly akin the degree of correspondence of the larger members may become a matter 

 of debate. Take, for instance, the three great phyla of living Pteridophytes, the 

 Ferns, Equlseta, and Lycopods. While the sporophyte as a whole in each of these 

 naay be accepted as homologous by descent with that of the others, the question 

 as to the true correspondence by descent of the leaves must still be open for dis- 

 cussion. It is a tenable view that the three phyla arose separately from a non- 

 foliar ancestry, and that the assumption of a foliar development, having in each 

 case a difierent habit, and a different relation to the sporangia, led to the dis- 

 tinctiveness of the three stocks. Opinion on the point of homology by descent of 

 the leaves of these Pteridophyta must at present remain in suspense ; but the case 

 is difiierent with the leaf of Pteridophytes as compared with the leaf of Bryo- 

 phytes : unless the whole morphological system of the time be in error, we shall 

 be right in maintaining that these foliar developments have been distinct in origin 

 from the first. 



Now all the foliar parts above quoted would in a system of merely formal 

 morphology fall into the category of ' leaves.' But if phylogeny be accepted as 

 the only real basis of morphology, we must be prepared to split up the category 

 based on mere time, place, and mode of origin, and to recognise in some cases 

 repetition of individual parts ; in others essential correspondence, but not individual 

 repetition, owing sometimes to transfer of developmental capability ; in other cases 

 again, a possibility of distinct origin by descent not actually proved ; and lastly a 

 reasonable certainty of distinct origin. The practical que.stion for the morpho- 

 logist is, having recognised these facts for himself, how is the matter to be best 

 made intelligible to others ? 



A reconsideration of the term ' homology ' will thus be necessary : is it to be 



