270 THE AMERICAN NATURALIST. [Vol. XXXVIll. 



ate pits occurs in only one case — A. robertiamitn. It is there- 

 fore n:\anifest that this genus is obviously of a more advanced 

 type than Cordaites from which it undoubtedly originated. 

 Agathis being represented by only one species, it is not possible 

 to locate it more definitely than to say that the 1-3-seriate dis- 

 position of its pits would place it in a position equivalent to that 

 occupied by Araiicatia ciinninghaini and therefore about three- 

 fourths way down the scale for that genus. This fact points 

 with much force to the idea that of the two genera, Agathis is 

 of relatively lower type. 



The Ginkgoales and the Coniferales as a whole, exhibit an 

 obviously higher type of development than the preceding group, 

 in consequence of the more pronounced tendency to segregation 

 of the pits which are now either elliptical or round, and never 

 hexagonal. This distinction is so clearly defined and constant 

 as to support the idea which gains force in other ways, that 

 Cordaites, Araucaria and Agathis are clearly related members of 

 a principal branch of the original stock, and that they therefore 

 diverge considerably from the particular line of descent within 

 which we find both the Ginkgoales and the Coniferales. 



The observations so far made apply altogether to the pits on 

 the radial walls. We may now pass to a consideration of their 

 relation to the tangential walls, a factor which does not call for 

 very extended discussion. This feature is found to apply to 

 71.7 f of all investigated species exclusive of fossils. It is 

 wanting in three species of Araucaria, representing 2.58 fr, in 

 Torreya, 2 species or 1.72 and in the entire second section of 

 Pinus to the extent of 28 species or 24.1 But the occurrence 

 of pits on the tangential walls, in common with those on the 

 radial walls, is a well known feature of the Sigillarias (49» P- 

 198), where their primitive character is well established, and we 

 can hardly doubt that their ultimate elimination in the higher 

 pines is the expression of a final phase in development, consist- 

 ent with the position usually assigned those plants. The 

 absence of pits from the tangential walls of certain Araucarias 

 and Torreyas, is to be interpreted as one of those sporadic ten- 

 dencies toward a higher type of development which never 

 become permanent in the same line, but which are to be met 

 with as one of the invariable features of evolution. 



