84 BIOLOGICAL ASPECT OF ALTERNATION 
The biological, or adaptation theory of antithetic alternation in the 
Archegoniatae, as embodied in the above paragraphs, was stated in my paper 
on alternation published in 1890.1 In 1894 came Strasburger’s Address 
on Periodic Reduction, delivered at the British Association meeting at 
Oxford.2) He there introduced as a structural basis of antithetic alternation 
that cytological distinction of the two generations which had already been 
suggested by Overton:? this at once gave a definiteness to alternation 
which it had never possessed before. He adopted a view similar . to 
that above stated, as a biological explanation of the rise and final dominance 
of the sporophyte, and pointed out how its gradual development can 
actually be traced, the first indications of it being apparently to be found 
in the Algae: they are to be sought in such post-sexual complications, 
connected probably with reduction, as are seen in O8¢dogonium. There 
is, however, no direct evidence of the origin of any Archegoniate form 
from any Alga: all that can be said is that, given such a multicellular 
body as the post-sexual stage of certain green Algae, the biological conditions 
of migration from water to land and of an amphibious life will sufficiently 
account for the further advances which are exemplified in land-plants. 
This is, then, the working hypothesis which will form the basis for 
our further enquiry. It will be necessary, however, to analyse the advance 
of the sporophyte, which is thus contemplated, from its simpler beginnings, 
and to consider the several factors which have been involved. Having 
done this, the enquiry will be made, what evidence there is in plants, 
living or fossil, that these factors of advance have actually been operative. 
‘The initial factor appears to have been “sterilisation,” that is, the delay 
of reduction by the conversion of cells which are potentially, and were 
ancestrally, sporogenous, into cells which serve no longer a propagative 
but a vegetative function. It will be readily seen that this is a necessary 
biological consequence of” any considerable increase in the number of 
spores ; and it has been pointed out above that such increase is a biological 
advantage, especially in those plants where a land-habit places restrictions 
upon increase in numbers by sexual propagation. The larger the number 
of spores the greater the powers of competition, and the greater the 
probability of survival, and of spread. On a biological theory, the nutrition 
of the increasing number was secured by the conversion of some of the 
potential germs to form a vegetative system, which should provide for 
nutrition and protection. It was naturally important that these tissues 
should be established in the individual before the sporogenous tissue 
which it is their function to nurse: and accordingly the time of spore- 
production was deferred, and a vegetative system, ultimately of great 
1 4nnals of Botany, 1890, p. 347. 
2 Annals of Botany, viii., 1894; Biol. Centralbl., Dec., 1894. A similar view has also 
been adopted by v. Wettstein, and embodied in his Handbuch aer Syst. Bot., Band ii, 
p- 13, published in 1903. 
+ Annals of Botany, vii. (1893), p. 139. 
