330 ABNORMALITIES OF THE LIFE-CYCLE [CH. 



gametophyte is well suited to moist conditions, while its ultimate function 

 of sexual reproduction cannot be carried out without the presence of external 

 water. It is in fact typically semi-aquatic in its nature, sharing many of its 

 characters with Algal types. The sporophyte is fitted by its more robust 

 habit as well as by the differentiation of its tissues for successfully enduring 

 exposure to relatively dry air, while this condition is essential for the dis- 

 persal of the spores. It may be held as probable that the circumstances of an 

 amphibial life, to which the early vegetation of the land must naturally have 

 been exposed, would tend to accentuate in their phylesis this characteristic 

 alternation of biologically different generations. But a further consideration 

 should also be taken into account. It is a fact illustrated in both of the 

 kingdoms of living things that the highest developments have been attained 

 by the diploid somatic phase. More especially is this so in the characteristic 

 vegetation of the land, in which a ventilation-system with stomata, a con- 

 ducting system of vascular tissues, and a high evolution of external form 

 are the specific features of the sporophyte. These are all either absent from 

 the haploid gametophyte, or are represented only in limited degree in some 

 of the most highly elaborated Mosses. The facts both in plants and animals 

 suggest that some higher potentiality lies in the diploid state, which may in 

 itself account in some degree for the ascendancy of the sporophyte, and for 

 its general dominance over the gametophyte in the vegetation of the land. 

 We may hold that it is able by its higher adaptability to seize upon and 

 make its own use of those sub-aerial conditions which are less easily met by 

 the haploid gametophyte. Whether or not this is a true picture of the origin 

 of the marked difference between the generations of the Archegoniatae, it 

 at least accords with the facts, and makes intelligible that high degree of 

 constancy, as well as of differentiation, which is seen in the alternate gene- 

 rations of the Archegoniatae. On the other hand, the more uniform circum- 

 stances of many Thallophytes would allow of such latitude as they actually 

 show in the adjustment of their chromosome-cycle to their somatic develop- 

 ment. Never having been exposed in the same way to the stress of sub-aerial 

 conditions, we may recognise that their life-cycle has not been standardised 

 to the same degree as that of the Archegoniatae. 



The further step of attempting to trace some exact source from which 

 this more highly standardised alternation of the Land-Plants may have 

 sprung would lead, in the absence of specific fact, to mere speculation. In 

 a treatise on the phylogeny of the Filicales such speculation can find no 

 place. It must suffice to state the plain facts of their alternation and the 

 modifications to which they are subject, and to compare them in general 

 terms with what is seen in the Thallophytes. But this should be done without 

 advancing those comparisons beyond the bounds of general Organography. 

 They should not be presented in the light of any definite phyletic suggestion 



