94 



SCIENCE 



[N. S. Vol. XXIX. No. 733 



to change our classification when we change 

 our theory of evolution. This follows log- 

 ically as a corollary of the main thesis, but 

 it is well to place our acceptance of it on 

 record, lest in our zeal for consistency we 

 may neglect it. No system of classification 

 should stand in its entirety after the theory 

 of evolution upon which it is based has 

 experienced any change whatever. The 

 two must be modified simultaneously, for 

 they are parts of a common system. 



What does the theory of evolution in- 

 volve to-day? It will be well to pause here 

 for a short enumeration of the principal 

 features of this theory so far as they bear 

 upon the question of classification. I need 

 scarcely remind you that for the purposes 

 of this discussion it is not necessary to 

 decide between the different schools of evo- 

 lutionists, since their differences are almost 

 wholly of such nature as to have little or 

 no bearing upon a system of plant taxo- 

 nomy. 



Elsewhere* I have enumerated the fol- 

 lowing dicta as involved in the theory of 

 evolution as applied to the vegetable king- 

 dom: 



1. In general the lower plants came into exist- 

 ence first. 



2. In general the higher plants sprang from the 

 lower. 



3. Higher plants are more complex than the 

 lower. 



4. Structures with many similar parts (homo- 

 geneous) are lower, those with fewer and dis- 

 similar parts (heterogeneoif^) are higher. 



5. Evolution is not aiii^yS upward, but often 

 involves degradation and ^generation. 



6. Evolution does not necessarily involve all 

 organs of the plant equally in any particular 

 period. 



7. One organ of a plant may be advancing while 

 another is retrograding. 



8. Upward development is sometimes through 

 an increase in complexity, and sometimes by a 

 simplification of an organ or a set of organs. 



' " A Synopsis of Plant Phyla," University of 

 Nelraska Studies, October, 1907, p. 1. 



9. In some cases particular structures become 

 more simple while the plants themselves become 

 more complex. 



10. Evolution has generally been consistent, and 

 when a particular progression or retrogression has 

 set in it is persisted in to the end of the phylum. 



11. Retrogression, once set in, usually persists, 

 and is not followed by a progression. 



12. Hysterophytic degeneration is persistent, and 

 the hysterophytic phylum never becomes holo- 

 phytic. 



13. In the first stages in the development of any 

 organ, whether upward or downward, the new 

 structures are not as fixed as they become later, 

 and in these earlier conditions there may be re- 

 versions to the ancestral structures, while later 

 such reversions do not occur. 



14. All plant relationships are genetic. 



15. Plants are related up and down the genetic 

 lines, and the system of plants to be quite natural 

 must recognize these phyla. 



"While these fifteen dicta are by no 

 means all that might be cited, they wiU 

 suffice for my present purpose. From 

 them the phyletic idea in taxonomy fol- 

 lows logically. Since all natural groups 

 must be phyletic, only that arrangement is 

 natural that recognizes these in their en- 

 tirety. It should no longer be permissible 

 on scientific grounds to propose a classi- 

 fication which is not phyletic. 



We may now profitably inquire as to the 

 origin of phyla, and to seek an answer as 

 far as it is possible to find one in general 

 terms and on theoretic grounds. Stated 

 philosophically, from what we know of the 

 relationship of organic beings it is obvious 

 that a phylum originates with the incom- 

 ing of a new idea. Stated structurally, 

 it has its beginning with the development 

 of a dominant morphological peculiarity. 

 Stated taxonomically, its initial point is 

 indicated by the appearance of a new char- 

 acter. 



Every phylum is the result of a develop- 

 ment which differs from that which pre- 

 ceded it because of the incoming of a new 

 dominant idea. This dominant idea was 

 manifested struetually by a divergence 



