232 SCIENCE PROGRESS 



traced, beginning in some distant geological period with a 

 homosporous ancestor which, although quite unknown at 

 present as a fossil, must have been essentially similar to a 

 modern fern. 



As a corollary of these discoveries, a discussion on the origin 

 of the seed itself has naturally arisen. Unfortunately in this 

 respect, the seeds of the Pteridosperms have all proved to be 

 highly evolved and complicated structures, and thus, in the 

 absence of features which might be considered primitive, they 

 hardly advance our ideas very definitely. Certain considerations 

 arising out of recent work have, however, been put forward 

 by Prof. Oliver, 1 Miss Benson, 2 and Miss Stopes. 3 The sub- 

 ject has also been fully discussed within the last few months 

 by Prof. Oliver, 4 in his presidential address to the Botanical 

 Section of the British Association at York. 



With regard to the ancestry of other groups of the Gymno- 

 sperms, such as the Coniferales and Ginkgoales, the evidence 

 derived from a study of fossil botany is by no means so clear at 

 present as in the case of the Cycads. This, no doubt, is chiefly 

 due to the fact that plant remains showing anatomical structure 

 are unfortunately very rare in the Mesozoic rocks. 



Both the Ginkgoales and the Coniferales begin to appear 

 before the close of the Palaeozoic period. The former have but 

 one representative living to-day — the Maidenhair Tree {Ginkgo 

 biloba, L.) of China and Japan, now almost extinct except where 

 cultivated. In the Mesozoic rocks we find abundant evidence 

 that this group, now reduced to a bare unit, formed at that time 

 a highly characteristic factor in the flora. A study of the living 

 plant has shown that in Ginkgo we have again a synthetic type, 

 in that it combines characters common to both Cycads and Ferns 5 

 on the one hand and to Cycads and Conifers on the other. 6 So 

 far, however, the study of fossil botany has not added materially 

 to what can be ascertained as to the ancestry of this group from 

 the still surviving member. 



When we turn to the Coniferales, the group which includes the 

 Pines and Firs, the fossil evidence is more valuable in this con- 

 nection. Yet botanists are not, so far, agreed as to whether 

 the Coniferae as a whole are monophyletic, i.e. derived from a 



1 Oliver, 1902, 1903. 4 Oliver, 1906. 



2 Benson, 1904. 5 Seward and Gowan, 1900, p. 146. 



3 Stopes, 1905. ti Scott, 1900, p. 521. 



