8o BIOLOGICAL ASPECT OF ALTERNATION 



The succeeding phase of the history has been one involving first 

 physiological and subsequently minute cytological considerations. The 

 study of the effect of external conditions on the succession of stages of 

 the developing organism, initiated by Klebs in 1889, led at once to the 

 recognition of the fact that in certain plants that succession is mutable 

 according to circumstances, while in others the succession is obligatory. 

 The distinction between different types of the life-cycle of organisms thus 

 established was found to coincide very nearly with the distinction drawn 

 by Celakovsky between "homologous" and "antithetic" alternation, and 

 thus his general position came to be greatly strengthened. But another 

 effect of the experimental test was to open up more definitely than before 

 the problem of origin of this obligatory succession of phases, in those cases 

 where it exists : it also accentuated the difference between the antithetic 

 or true alternation, and those other appearances which bear a superficial 

 resemblance to it. But meanwhile the question of the rise of the neutral 

 generation was being approached also from the point of view of adaptation, 

 and a theory of its origin in an amphibious mode of life, which it will be 

 the object of this chapter to develop, was already being advanced as an 

 explanation of the progress and final dominance of the sporophyte in the 

 plants of the land. It is clear, however, that adaptation would only account 

 for its advance, not for its ultimate origin. This amphibious theory was 

 based upon physiological considerations, together with closer observation 

 of the origin of the sporogenous cells, their limitations, and their relation to 

 the tissues which are merely vegetative. Lastly, more careful observation 

 of the details of sexuality and of spore-production led to the generalisation 

 on the basis of minute nuclear structure : this put the cytological cachet, as 

 well as a structural check upon the conclusions already drawn. But the 

 existence of a chromosome-difference between the two generations turns 

 attention afresh to the question of the ultimate origin of the obligatory 

 succession of phases : it suggests that the origin was in sexuality, and in 

 those post-sexual complications which are so frequently the consequence 

 of nuclear fusion. Naturally these several phases of the study of alternation 

 have overlapped one another, and proceeded in some degree coincidently. 

 One of the interesting features in the history is that their results have 

 often run so nearly parallel as to yield a high degree of mutual support. 



It has been remarked above that up to the time of Celakovsky the 

 study of alternation was on the basis of form ; but it is now clear that the 

 merely formal comparison of different organisms, or of their successive 

 stages one with another, cannot suffice for the full solution of the question 

 as to the real nature of alternation. The case of the structurally similar 

 but cytologically distinct generations of Dictyota show this, while the differ- 

 ence of the propagative organs which they bear confirms the distinctness 

 of the two generations. In the Florideae also, there are no definite structural 

 details which serve as formal differentiating characters between the pre-sexual 

 and the post-sexual stages. Such examples will probably be multiplied as 



