FACTORS OF ORGANIC EVOLtTlONi 455 



of growth are disposed in like fashion; and it is significant to observe 

 that in the compositse, which is considered to be the latest and highest 

 general type of plant form, the rotate or centrifugal arrangement is 

 most emj)hatically developed. The circular arrangement of parts is the 

 typical one for higher plants, and any departure from this form is a 

 specialization, and demands explanation. 



The point I wish to urge, therefore, is the nature of the obvious or 

 external divergence of plant-like and aiiimal-like lines of ascent. The 

 significance of the bilateral structure of animal types is well understood, 

 but this significance has been drawn, so far as I know, from a compar- 

 ison of bilateral or dimeric animals with rotate or i^olymeric animals. 

 I want to put a larger meaning into it by making bilateralism the symbol 

 of the onward march of animal evolution and circumlateralism (if I may 

 invent the term) the symbol of plant evolution. The suggestion, how- 

 ever, applies simply to the general arrangement of the parts or organs 

 of the plant body, and has no relation whatever to functional attributes 

 or processes. It is a suggestion of analogues, not of homologues. We 

 may therefore contrast these two great lines of ascent, which, with so 

 many vicissitudes, have come up through the ages, as Dipleurogenesis 

 and Centrogenesis. 



The two divergent directions of the lines or phyla of evolution have 

 often been the subject of comment, but one of the sharpest contrasts 

 between the two was made in 1884 by Cope, when he proposed that the 

 vegetable kingdom has undergone a degenerate or retrogressive evolu- 

 tion. "The plants in general," he then wrote, '-in the persons of their 

 protisfc ancestors, soon left a free- swimming life and became sessile. 

 Their lives thus became parasitic, more automatic, and, in one sense, 

 degenerate." The evolution of the plant creation is, therefore, held to 

 be a phenomenon of catagenesi s or decadence. This, of course, is merely 

 a method of stating a comj)arison with the evolution of the animal line 

 or phylum, and is therefore of the greatest service. For myself, how- 

 ever, I dislike the terms retrogressive, catagenetic, and the like, as 

 applied to the plant creation, because they imply intrinsic or actual 

 degeneracy. True retrogressive or degenerate evolution is the result 

 of loss of attributes. Cope holds that the chief proof of degeneracy 

 in the plant world is the loss of a free-swimming habit, but it is possible 

 that the first life-plasma was stationary ; at any rate, we do not know that 

 it was motile. Degeneracy is unequivocally seen in certain restricted 

 groups where the loss of characters can be traced directly to adaptive 

 changes, as in the loss of limbs in the serpents. Eetarded evolution 

 expresses the development of the plant world better than the above 

 terms, but even this is erroneous, because plant types exhibit quite as 

 complete an adaptation to an enormous variety of conditions as animals 

 do, and there has been rapid progress toward specialization of structure. 

 As a matter of fact, the vegetable world does not exhibit, as a whole, 

 any backward step, any loss of characters once gained, nor any station- 



