92 BOTANICAL GAZETTE [august 



grounds of comparative convenience; to make the suggestion 

 convincing in any group where general reduction is believed to have 

 occurred it will be necessary to prove that the sum of nutrition, 

 from whatever source, has diminished in the course of descent and 

 that the reduced spore output has been the result. Until this has 

 been shown to have occurred in any case, there seems no sufficient 

 reason to accept as more than a quite open hypothesis any sugges- 

 tion of general reduction of the sporophyte" (Bower 7). "The 

 hypothesis of relative primitiveness has then logically prior claim 

 and must be accepted as a working theory until good grounds can 

 be given for preferring that of reduction'' (Tansley 31). 



With the development of a dendritic habit among certain groups 

 of the Lycopods came greater capacity for spore production, and 

 this was followed up structurally in several ways, for the multiplica- 

 tion of spores in a homosporous plant, with equal chances of dis- 

 semination, must be considered an advantage as long as these spores 

 can be properly matured, and in many forms it was probably not 

 merely an advantage but a necessity to produce spores to the limit 

 of their capacity in order to maintain themselves in the struggle 

 for existence. Unfavorable environmental conditions, such as 

 slow unfavorable climatic changes, may be expected to decrease 

 the capacity for spore production, and this among plants that have 

 been producing spores to the limit of their capacity will inevitably 

 lead to one or more of several possible forms of reduction. There 

 may be reduction in the number or the size of the sporangia 'or 

 sterihzation of some of the potential sporogenous tissue or arche- 

 sporium. 



During geological time there have been great periods of marine 

 transgression, punctuated by periods of regression or continental 

 emergence, usually accompanied by orogenic disturbances, and 

 each of these great cycles of earth experience constitutes a geological 

 period. Changes in the distribution and altitude of land masses 

 toward the close of each of these periods were attended by climatic 

 changes which were sometimes profound. They usually tended 

 somewhat toward aridity, accompanied as a rule by a reduction 

 of the mean annual temperature. To geologists the evidence 

 supporting such facts is today so eminently satisfactory that they 



