534 



SCIENCE. 



[N. S. Vol. XX. No. 512. 



ing phyletie ideas, can only be properly 

 understood by glancing back at the history 

 which has produced it. So long as species 

 were regarded as the individual results of 

 creative power, the complexity and variety 

 of their form were relegated to the arcana 

 of the divine mind, and organic nature 

 presented the aspect of a series of isolated 

 pictures; any similarity which these might 

 show was to be regarded as indicative of 

 the underlying divine plan. Now that 

 species have been threaded together by 

 evolutionary theory into developmental se- 

 quences, they, like the ribbon of a cinema- 

 tograph, present phyletie history to the 

 mind with all the vividness of a living 

 drama. "While monophyletic views held the 

 field, this seemed comparatively simple; 

 but the conclusions thus arrived at in plant 

 morphology were often palpably improb- 

 able. Such difficulties, together with the 

 substantiation of examples of parallel de- 

 velopment on a sound comparative basis, 

 led to the modification of monophyletic 

 views, and opened the way for less cramped 

 conceptions. It is now customary to con- 

 template the plural origin of such leading 

 features as sexual differentiation, foliar de- 

 velopment, heterospory, the seed habit, as 

 well as a host of minor characters. On 

 such examples Ave base a general belief that 

 similar structures may be arrived at by 

 divers evolutionary routes. It is this con- 

 ception of polyphyleticism that we must 

 make clear in our descriptions, if not even 

 in our terminology. 



It will be objected that to carry through 

 a method of designating by the same term 

 only such parts as are shown to be of com- 

 mon descent would produce unwieldy re- 

 sults. Doubtless this is true, but in the 

 terminology of a science it is not so much 

 convenience, as truth and clearness which 

 should be the aim. The choice is open to 

 us either to make the terminology strictly 

 phyletie throughout, which would certainly 



be cumbrous, though it would reflect the 

 true position, or, putting phyletie distinc- 

 tions in the background, to use terms in a 

 more or less comprehensive sense, even 

 grouping together things which we know 

 to have been distinct in phyletie origin. 

 Such a comprehensive sense is conveyed by 

 the expression 'homology of organization,' 

 which, as Goebel points out, 'has only to do 

 with phylogeny in so far as it recognizes a 

 common capacity for development derivable 

 from undifferentiated ancestors.'* 



This is, indeed, a collective term for the 

 results of parallel development; it suffers 

 from the danger of suggesting some ideal 

 type, or pattern, towards which evolution 

 has tended. 



For my own part, I think it matters little 

 what our terminology be, or what the 

 separation of categories of parts, provided 

 we attach clear meanings to the words we 

 use, and select those Avords as naturally 

 conveying that meaning. For instance, if 

 we fully realize that the Avord 'leaf is used 

 in a sense which is not phylogenetic, but 

 merely descriptive of those lateral append- 

 ages on the shoot Avhich are produced 

 exogenously, and in acropetal order, then 

 let it remain, ranking as an expression of 

 'homology of organization.' But the ap- 

 pendages thus included may for clearness 

 be conveniently divided into 'phyllomes' 

 on the sporophyte and 'phylloids' on the 

 gametophyte, as indeed I suggested some 

 years ago. Nevertheless, these again are 

 not phyletie unities; they include parts 

 with distinct histories which have already 

 been recognized in the gametophyte, while 

 for the sporophyte a more advanced state 

 of the science will probably provide defini- 

 tions. Meanwhile Ave consent to a com- 

 promise in grouping these together ; but the 

 only condition upon which this can be 

 safely done is the clear knowledge that this 

 is a compromise by which we secure a cer- 



* ' Organographie,' English edition, p. 19. 



