7IO THE AMERICAN NATURALIST. [Vol. XXXVIIl. 



which such a feature is fully expressed, and it thereby forms the 

 basal member of another series. From the opposite point of 

 view, it has been shown that the occurrence of two-seriate pits 

 in Pinus and others of the Coniferales, as well as in Ginkgo, 

 points to a common origin for such genera in a type with multi- 

 seriate, hexagonal pits, and that both Agathis and Araucaria 

 must likewise center in the same generalized form. This grad- 

 ual convergence is justified on other grounds, and the genus 

 Poroxylon among known forms most nearly fulfills the require- 

 ments of the case. We may therefore look upon it as lying 

 between the Cycadofilices and all the higher gymnosperms, giv- 

 ing rise to two lines of descent, the first of which embraces the 

 Cordaitales as already described, while the second shortly divides 

 once more. This secondary division gives rise on the one side 

 to the Ginkgoales, and on the other to the Coniferales. The 

 anatomical data already discussed when view^ed collectively show 

 that the general sequence wdthin the latter would be (i) the 

 Taxoideae, (2) the Taxodiinae, (3) the Cupressineje, (4) Abies, 

 (5) Tsuga, (6) Pseudotsuga, (7) Larix, (8) Picea and (9) Pinus, 

 of which one division (II) represents the highest type of devel- 

 opment. The sequence of species for each genus cannot always 

 be determined with a full measure of satisfaction, and these 

 difficulties may possibly be made clear by reference to a particu- 

 lar case. The succession of the two species of Sequoia is diffi- 

 cult to determine on purely anatomical grounds, but the general 

 tendency of the facts already recited is to give to S. scmfer- 

 virens the more primitive position, a view w^hich is sustained by 

 its palaeontological history. 



The relations brought out in the foregoing studies, and the 

 conclusions reached, may be made more obvious without the tedi- 

 ous method of a detailed discussion by reference to the accom- 

 panying table of anatomical data, which substantially summarizes 

 all the results derived from the study of particular structures. 



