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PREFACE 



IN the year 1874 apogamy was discovered in Ferns by Farlow : and in 

 1884 instances of apospory in Ferns were demonstrated before the Linnaean 

 Society of London by Druery. These events stimulated a fresh enquiry into 

 the nature and origin of Alternation in Archegoniate Plants. My own 

 observations on apospory confirmed my interest in this question : it seemed 

 to me probable that some biological cause had determined the prevalence 

 and constancy of the alternation, to which apogamy and apospory appear as 

 occasional exceptions. The theory was entertained that the change of 

 conditions involved in the invasion of the Land by organisms originally 

 aquatic had played a prominent part 'in the establishment of those 

 alternating phases of the life-cycle which are so characteristic of Archegoniate 

 Plants. As early as 1889 I had already written several chapters of a 

 treatise on this subject : but the necessary facts were found to be then so 

 imperfectly known that the work was abandoned, and instead of a full 

 discussion of the matter, the Biological Theory of Antithetic Alternation 

 was briefly stated in a paper published in the Annals of Botany in 1890 

 (vol. iv. p. 347). The main position of Celakovsky in discriminating 

 between Homologous and Antithetic Alternation was adopted ; but the 

 latter type, as seen in Archegoniate Plants, was recognised as having been 

 fixed and perpetuated in^accordance with the adaptation of aquatic organisms 

 to a Land- Habit. The Studies in the Morphology of Spore-producing Members 

 were then entered upon as preliminary investigations to elucidate the facts 

 requisite for a more full statement, and they were published in five parts, 

 from 1894 to 1903. Meanwhile, in 1894 Strasburger contributed to the 

 Meeting of the British Association in Oxford his paper on the " Periodic 

 Reduction of Chromosomes." He brought together a wealth of facts 

 establishing the cytological distinction of the alternating generations, and 

 his theoretical position was virtually identical with that of my paper of 

 four years earlier. 



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