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. 



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. 



