THE HISTORY OF PLANT LIFE. 29 



their most elementary members ; and thus it becomes possible 

 to ascertain at what points suppression has occurred and the 

 normal succession been broken. 



We are perhaps not far wrong in the assumption that the 

 general line of descent began with the green algae as the first 

 clearly defined type. From these aquatic forms, which were, 

 no doubt, at first dominant, if not the exclusive forms, am- 

 phibious types appeared, leading eventually to terrestrial forms, 

 as represented by the mosses and liverworts, plants which 

 clearly show their derivation from an aquatic ancestry when 

 they enter upon their reproductive phase, and especially in the 

 development of an algoid protonema. But. here we encounter 

 one of the so-called missing links, since, although the approach 

 of the alga to the moss, and of the moss to the alga, is well 

 defined, the intermediate stages are as yet unknown. And so 

 again the thallus of the liverwort reappears in the prothallus 

 of the fern, and as these latter lead on to higher types, we find 

 undoubted evidence of relationship, the exact bond of which is 

 as yet wanting. One of the most significant facts, however, 

 is to be found in the evidence of an aquatic ancestry, which 

 extends through all grades of development in terrestrial plants 

 until the Cycads are reached ; I refer to the occurrence of 

 motile spermatozoids, the significance of which can scarcely be 

 misunderstood. 



It is highly probable that further light respecting these ob- 

 scure problems will be gained as our knowledge of living forms 

 advances ; but as many of the existing gaps in the biological 

 tree have resulted from suppressions which must have occurred 

 in remote periods of the earth's history, it is peculiarly within 

 the province of palaeobotany to discover the necessary data, and 

 thus supply the missing links in the chain of plant life, in order 

 that we may have a clear and complete explanation of the rela- 

 tions of the various great groups of plants. And it is among 

 those forms which are now extinct that our search must be 

 prosecuted with the greatest hope of success. That this is 

 within the limits of possibility has been abundantly shown by 

 the investigations of recent years, and possibly no better illus- 

 tration could be had than the evidence derived from extinct 



