THE NATURE OF ALTERNATION, ETC. 343 



difference which has been established is causally related 

 to the origin of the second generation. It is quite possible 

 that it may have been simply coincident with the germina- 

 tion of the zygote in situ without a previous rejuvenescence 

 or division into swarm spores. 



All progress in our knowledge of the relationships of the 

 various groups of plants may aid in arriving at some deter- 

 mination as to the early course of evolution of the sporophyte. 

 But in addition to the comparative morphology of existing 

 forms, the experimental study of the Green Algae, the 

 Liverworts, and the simpler Vascular Cryptogams may be 

 expected to yield important evidence on this question. The 

 attempt to induce the zygote in CEdogonium or Coleochcete 

 to develop directly into a sexual plant, and the experimental 

 study of apogamy and apospory may be mentioned as bear- 

 ing on the truth of the homologous theory. On the other 

 hand, the experimental causation of sterilisation of spore- 

 producing tissue of simple sporophytes by hindering the 

 nutrition of the latter, as suggested by Atkinson, 1 might 

 yield results confirmatory of the antithetic theory. Such 

 induced changes, though merely " morphogenetic possi- 

 bilities," would afford, if used with care, satisfactory guides 

 to speculation. The fact that the changes are in many 

 instances sudden and discontinuous need not in the light 

 of recent work on variation exclude them from this use. 



In concluding this outline of the history of the theory 

 of alternation of generations in plants, hope may be ex- 

 pressed for a partial solution in the future, though this may 

 be far distant. The clear recognition of the openness of 

 the question is the best safeguard for the facts which sup- 

 port one or other theory having their proper weight accorded 

 to them. The spirit in which such investigation should be 

 carried on has been well expressed by Dr. Scott : " Let us 

 in the presence of the greatest mystery in the morphology 

 of plants keep an open mind, and not tie ourselves down 

 to assumptions, though we may use them as working 

 hypotheses ". 



1 Atkinson (2), p. 180. 



