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 Aznals 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 Studzes 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. 
