236 AMPLIFICATION AND REDUCTION 
a thing with pronounced characters, otherwise it would not be held as 
typical: there will then be an inherent probability that allied forms ‘would 
range themselves as reductions from such a type. On the other hand, 
in series which have really been ascending series, the original forms would 
not be prominent as types, and so would not be likely to command 
attention. 
Commonly it has been on a basis of simple comparison that phyletic 
series have been traced; but it is plain that apparent sequences should 
be checked according to other considerations than those of mere formal 
comparison. The most important of such checks is that of physiological 
probability, or even in some cases possibility. In those phyla where the 
organisms are relatively isolated, and the wide gaps in the series make 
comparisons less certain, such checks are specially necessary, and in none 
more so than in the Pteridophyta. 
There is overwhelming evidence that the homosporous state was the 
original condition of all the known phyla of Pteridophytes, as it is ‘the 
uniform condition of all the Bryophytes. It may be assumed that it was 
while still in this condition that the leading characters of their several 
sporophytes were established, though in many of them the heterosporous 
state supervened at a later date. This brought with it complications of 
the factors which originally determined the form of the sporophyte. It 
is desirable to avoid any confusion of these later factors with those which 
determined the character of the sporophyte in its more primitive homo- 
sporous state. It will be best to put them on one side for the moment, 
and to confine the attention at first to the simpler problem of the evolution 
of the homosporous types: for this will be found to give a better insight 
into the principles relating to amplification and reduction, and the part 
which they respectively played in the evolution of the primitive 
sporophyte. bd 
According to the adaptive theory of alternation, as stated in Chapter VI., 
the extended development of the sporophyte acted as an offset to those 
obstacles to fertilisation which faced aquatic organisms as they extended 
to a land habit. Where all germs are alike (homosporous), the larger the 
number of them produced the greater the probability of survival: thus 
selection would favour those with the highest spore-output. But to secure 
a high output of spores there must be an adequate supply of nutritive 
material: thus a condition of any extension of spore-output will be a due 
nutritive supply; and, conversely, any diminution of nutritive supply will 
reduce the output. The two systems, that of nutrition and that of 
propagation, will thus tend to vary together as regards amplification or 
reduction. And since in homosporous forms the highest chance of survival 
and of spread lies with those organisms capable of the highest numerical 
propagation, we should naturally anticipate that in them, other things 
being equal, a general progressive amplification would have. the upper 
hand. 
