242 SCIEXCE PROGRESS. 



In no branch of the subject have these dangers been 

 more apparent than in the study of the Thallophytes ; 

 though, possibly, examples are oftener quoted from among 

 the Flowerinof Plants, because the facts seem easier to 

 obtain and more obvious. 



The morphological doctrine known as the alternation of 

 generations is a case in point with reference to the former ; 

 while the disputes as to the relation of highly adapted — 

 specialised — plants, such as Xerophytcs, to their environ- 

 ment afford examples in the latter connection. 



It being admitted that the alternation of generations 

 as matter of fact exists in the BryopJiyta and Ptcridophyta, 

 the attempt to read the dogma into the life history of the 

 Thallophytes has been made so often and so persistently that 

 the unwary or insufficently informed are liable to suppose 

 it is more successful than is really the case, while even more 

 cautious speculators seem to be persuaded it is necessary 

 — a consequence of persistence as it were. 



Again it occurs, largely owing to misapprehensions 

 of the real difficulties which Darwin saw so clearly, that 

 some botanists regard the case for a Xerophyte — as a 

 particular example — being structurally the direct outcome 

 of the persistent actions of the environment, as much 

 more nearly capable of demonstration than the present 

 condition of science allows. 



Morphologists divide the life-cycle of a Moss into two 

 parts. The Moss-plant [Gametophyte), arising as a vegeta- 

 tive outgrowth from the protonema of the germinating 

 spore, ending in the production of the oosphere, etc., 

 being the one ; the Sporogonium [Spoivpkyte) constituting 

 the other. 



If we ask why two generations.'* Why the profound 

 structural differences between the protonema and the leafy 

 stem, their sharp segmentation, entire change of growth, 

 cell-division, etc., do not justify our making a third break 

 here in the life-cycle ? The morphologist replies because the 

 organic separation between embryo and gametophyte on 

 the one hand, and between protonema and sporophyte on 

 the other, are so complete that in each case a breach of 



