PHYLOGENY 431 



were, doubtless, cases in which it would have been difficult, or im- 

 possible, to draw a line between a heterosporous pteridophyte and a 

 seed plant. If the retention of the megaspore makes the sporangium, 

 with its contained megaspore, a seed, while a sporangium which sheds 

 its megaspore, even at an advanced stage of development, has not yet 

 reached the seed condition, a single individual might be partly fern 

 and partly seed plant. Such a condition sometimes occurs in Se- 

 laginella, and may have occurred rather frequently as the early 

 gymnosperms were evolving from the heterosporous pteridophytes. 



During this period of transition, it would not be surprising to find 

 the leaf remaining at the fern level. In very recent times, numerous 

 varieties of apples, of various aspect, have arisen, while the leaves 

 remained about the same. In the same way, the fern leaf was re- 

 tained by the early gymnosperms. 



In vascular anatomy the CycadofiHcales are more advanced than 

 the ferns with which they were associated. Circular pits are charac- 

 teristic of the xylem of higher seed plants, while a scalariform marking 

 is equally characteristic of ferns. The known CycadofiHcales have 

 quite generally progressed beyond the scalariform stage, but we 

 should expect to find it in their seedHngs, for even the living cycads 

 pass through a scalariform stage, and Stangeria does not get beyond 

 it, except in a few tissues. The genus also retains a very fernlike leaf. 

 This is instructive, for it shows that the fern leaf and vascular anat- 

 omy may be retained after the seed habit has become estabhshed. 

 Stangeria was described as a fern, and even assigned to the genus 

 Lomaria, until its seeds were discovered. And so the lower gymno- 

 sperms and plants, which had almost, but not quite, reached the 

 seed condition, would naturally resemble each other so closely that 

 they could not be distinguished by vegetative characters: and the 

 resemblance would be due to genetic relationship. 



Where complete life-histories are known, we believe that recapitu- 

 lation is a definite help in solving relationships, especially in nearly 

 related forms. Here, we should lay the greatest stress upon the 

 origin and development of heterospory and the seed. Next, we 

 should rank the evidence from vascular anatomy, like the occurrence 

 of spiral, scalariform, and pitted structures, and exarch, mesarch and 

 endarch bundles. The behavior of leaves, hke the evidence from 



