PHYLOGENY 429 



No botanist will deny that heterospory has arisen independently 

 in various groups of plants, and that it has arisen in the same way in 

 the most diverse cases — by the disorganization of young megaspores 

 which have been absorbed by growing megaspores. 



We have already remarked that the Cycadofilicales were mis- 

 taken for ferns until their seeds were discovered, and it is possible 

 that the discovery of seeds may transfer more of the oldest recog- 

 nized members of the Filicales to the Cycadofilicales. 



In the angiosperm flower there seem to be some rather definite 

 lines of evolution — from spiral to cyclic arrangement of parts, and, 

 in the cyclic forms, from pentacyclic to tetracyclic, from hypogyny 

 to epigyny, etc. On the basis of these tendencies, the Archichlamy- 

 deae have been arranged in a line from the Amentiferae to the Um- 

 belliferae; the Sympetalae, in a line from Ericaceae to Compositae; 

 and the monocotyls, in a line from the Pandanales to the Orchids. 

 This does not mean that each order must be derived from the one 

 below it; but it is probable that some tetracyclic flowers have come, 

 by direct descent, from pentacyclic flowers. However, the pos- 

 sibihty that similar structures may be due to parallel development 

 must always be recognized. It may be that most of the forms in the 

 three great lines of angiosperms have come, by parallel develop- 

 ment, from a few great centers like the Ranunculus — Rose — Legume 

 plexus in the Archichlamydeae. 



Similarly, there are lines of evolution in the gymnosperms, which 

 does not mean that each family is derived from the one ranked be- 

 fore it, or even that the genera within a family are mentioned in 

 genetic sequence. 



The sequence of families in an order, and the sequence of genera 

 in a family, is generally based upon the evolution of some one im- 

 portant structure. For example, in the Fucaceae, Fucus, with eight 

 eggs in the oogonium, is placed first, while forms with four, two, and 

 only one egg, come later. There can be no doubt that reduction in 

 the number of eggs in this family is an evolutionary tendency, and, 

 in this respect, Fucus is the most primitive. But, in the most primi- 

 tive form of plant body, conceptacles cover the entire plant. On this 

 basis, Hormoseira should stand at or near the beginning, although 

 only four of its eight nuclei become nuclei of functional eggs. Fucus, 



