1 64 SCIENCE PROGRESS 



These are some of the fundamental points in the theory, which is submitted by 

 the author as a working hypothesis, since the supposition that the alternation of 

 generations, shown by bryophytes and pteridophytes, really is antithetic in origin, 

 appears to lie outside the realm of possible proof under present conditions. 



The book is divided into three parts, of which the first is devoted to a statement 

 of the working hypothesis. Here various subjects are dealt with in turn ; in each 

 chapter an excellent summary is given of the present state of knowledge with 

 regard to one subject, and the facts elicited are then discussed from the point of 

 view of their bearing on the biological theory of alternation. Among the subjects 

 discussed in this part of the book are the cytological distinction of the alternating 

 generations ; apogamy and apospory ; the morphology of sporangiophores and 

 sporophylls ; sterilisation ; embryology and the theory of recapitulation ; vascular 

 anatomy. 



The second part of the book contains a detailed description of the sporophyte 

 in the different groups of Bryophyta and Pteridophyta, taken serially, and treated 

 intimately as regards the form, anatomy, and development of the vegetative and 

 reproductive parts. In the different phyla careful comparisons of series of allied 

 forms are entered into for the purpose of obtaining views on their phylogenetic 

 sequence, the morphological status of their organs, etc. 



The third part is of the nature of a summary of the preceding portion of the 

 volume, treated in relation to the working hypothesis, and as a source of some 

 general phyletic and morphological conclusions. Besides other subjects this part 

 contains a comparative description of the embryogeny of the pteridophytes, and 

 a chapter on heterospory and the seed-habit in their biological aspect. 



A few conclusions regarding the Pteridophyta may be quoted here. In the 

 early history of the sporophyte the axis was pre-existent to its appendages, which 

 subsequently arose by enation from its surface, and probably independently in 

 different phyla. Foliage-leaves have originated from sporophylls or bracts, the 

 sporangia or other spore-bearing organs which they bore or subtended having 

 disappeared ; this belongs to the theory of the strobilus. Sporangiophores are 

 parts sui generis, and are not the result of modification or replacement of any other 

 sort of appendage. The primitive type of sporophyte for the vascular cryptogams 

 had an upright axis possessing apical growth and radial symmetry, and producing 

 lateral appendages in acropetal succession. These appendages were from the first 

 of two kinds, viz. bracts or leaves and spore-producing members, and were closely 

 associated together. The spore-producing members include sporangia, sori, and 

 sporangiophores. 



It will not be possible to give any full discussion of the conclusions here ; but 

 a few remarks on one or two points may be made. 



With regard to the view that the uncertainty of sexual reproduction on land 

 puts a premium on an alternative mode of reproduction suited to dry conditions, 

 it may be suggested that a biological explanation ought perhaps to give the prior 

 place to dispersal. The dissemination of zygotes by water-currents having been 

 lost, a means of wide dispersal becomes important, and multiplication without 

 dispersal would not effectively meet the case. 



The sporophyte is regarded as having been originally dependent on the parent 

 prothallus (p. 218). The opposite view that the sporophyte generation may in the 

 first instance have been independent of the gametophyte for its nutrition is perhaps 

 dismissed in rather too summary a fashion (p. 226). The first is probably the 

 natural view to any one committed to anthithetic alternation ; but it may be pointed 

 out that, if the hypothesis of homologous alternation be preferred, the original 



