80 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 
6x_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 viewof adaptation, 
-and_a theory of its origin in_an amphibious.-modeaf life, which it will be 
Aa Tm cree a ha Rg ees eee 
explanation of the progress and final dominance, of the sporophyte in the 
“Diants oF the land. It is clear, however, that adaptation would only account 
Yor iw SENENCS ROE fon 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_chiamesome,difieance—helween the two generations turns 
attention stl origin” of ‘the obligato 
Succession. of phases : it suggests that the origin was in sexuality, _andin— 
_those_post-sexual complications whieh—a uently the consequence 
_of nucleae-feston. Naturally these several phases of the stu y 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 parallek as to yield a high degree of mutual support. 
It has been remarkéd 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 Dzctyofa 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 
