340 SCIENCE PROGRESS. 



with those previously stated by Celakovsky and Bower as the 

 result of morphological comparison. 1 



The nature of alternation has also been considered by 

 Macmillan 2 and Fry, 3 but a consideration of these and other 

 views would not aid in the elucidation of the main problem, 

 since they differ mainly on points of detail. 



Scott 4 has given a most suggestive review of the two 

 alternative theories of alternation in his presidential address 

 to the Botanical Section of the British Association. The 

 critical nature of this work necessitates a reference to the 

 original, but the important bearing of his clear restatement 

 of the homologous theory upon the present state of opinion 

 on the nature of alternation must be pointed out. 



CONCLUSION. 



To attempt a general discussion of the nature of alterna- 

 tion of generations and its relation to the successions of 

 forms which have been distinguished as generations among 

 the Thallophyta is not within the scope of this article. 

 Some of the factors in the problem will, however, be 

 referred to, and some lines of investigation, the results of 

 which may be expected to aid in its solution, indicated in 

 conclusion. 



A study of the literature has made it evident that the 

 facts have been interpreted very differently by the investi- 

 gators who have discovered and discussed them. If we 

 attempt to determine what is the essential distinction be- 

 tween the theories of homologous and antithetic alternation, 

 the conclusion arrived at will, it appears to me, be of this 

 nature. On the homologous theory, the sporophyte is to 

 be traced back to a generation of originally independent 

 individuals similar to those from which the gametophyte has 

 arisen, the almost invariable alternation and the permanent 

 or temporary dependence of the spore bearing on the sexual 

 generation being subsequent adaptations. On the antithetic 

 theory, the sporophyte is not derived from free-living in- 



1 Strasburger (2), (3). 2 Macmillan. 



3 Fry. 4 Scott> 



