360 Causes and Course of Organic Evolution 



of tlie Cordaitales into two additional branches beside the 

 Gnetales, and which became increasingly dominant through 

 Jurassic and cretaceous times, while as a coeval event the stock 

 genera like Cordaiies, Dorycordaiies, and allies were dying out. 

 They constitute the two groups already named, the former of 

 which or Ginkgoales we will now consider. In doing so we 

 may be aided if we consider living material, and then work 

 back to tj^pes that seem to connect with the parent stock. 



Ginkgo hiloha, the maidenhair tree of China and Japan, 

 is the sole surviving representative of a genus that can be 

 traced through the tertiary rocks into the Jurassic strata of 

 the mesozoic age, or even according to Sapor t a back into the 

 pennian. The greatest number of species apparently flour- 

 ished during the late cretaceous or early tertiary times. Like 

 the more ancient Cordaitales the stem grows indefinitely, 

 and shows similar porous fiber elements. The leaves, in shape 

 and veining, can well be connected by genera like Psygmo- 

 phyllnm from the permian and Dicranophylluni from the car- 

 boniferous rocks with cordaital forms. But in the small num- 

 ber of prothallial cells in the pollen grain, and in the solitary 

 succulent seeds, the Ginkgoales show specialized details, at 

 the same time that they retain the evidently ancient peculi- 

 arity of producing large polyciliate spermatozoids. 



The genera Ginkgophyllum, Baiera^ and Noeggerathia, in 

 part from the structure of their leaves, and, when known, from 

 that of their flowers, were apparently related genera of the 

 family that occur in permian rocks, reach their climax of devel- 

 opment during cretaceous times, and are wholly or largely 

 extinct by early tertiary times. 



But a third family now to be considered, and with which 

 Ginkgo was formerly united, is that of the Taxaceae or yew 

 alliance. Fron what we know of fossil remains it may be 

 said that of the eight living genera {Saxegotheay Microcachrys, 

 Dacrydium, Phyllocladus, Podocarpus, Cephalotaxus, Torreya, 

 and Taxus) some are unknown in the fossil state, others like 

 Podocarpus and Torreya can be traced back to the eocene or 

 even cretaceous age, while some that have been described 

 as belonging to Taxus are even more ancient. 



In all of these except PhTjllocladus, there is a tendency for 

 the leaves to show condensed, simplified, and more leathery 

 or resistant texture than in the Ginkgoales, the leaves being 

 often lanceolate to linear or even tooth-like. Unfortunately 

 we still know little regarding the flowers and fruits of the fossil 



