10-5G REPORT— 1898. 



still unknown ; it is not even certain that the cells of the aposporously produced 

 prothalli possess the half number of chromosomes, and those of the apogamously 

 produced sporophytea the double number, though this may be assumed to be 

 probable. Apospory at least might be readily explainable by such a nuclear 

 change. 



With regard to apogamy, however, some general conclusions may fairly be 

 drawn even in the absence of observations on the nuclei. For whatever change 

 may take place in the latter, it is certain that the transition from prothallus to 

 sporophyte, or from prothallold to sporophyte tissue, takes place without relation 

 to the sexual fusion, and is so far comparable to an ordinary variation. Further, 

 it is to be noted that the change takes place, so far as the conditions are known, 

 when, by preventing the access of fluid water, fertilisation is delayed, and when 

 in other ways the conditions approach those favourable to the sporophyte rather 

 than the gametophyte. These modifications of the conditions are of the kind to 

 which aquatic organisms would be exposed on assuming a terrestrial habit. It is, 

 therefore, possible to view the changes which take place in prothalli under these 

 circumstances, not as reversions, but as indications of the capability of the gameto- 

 phyte to assume the characters of the sporophyte under suitable conditions. If there 

 is any truth in this way of regarding the facts of apogamy, they become of value 

 in enabling us to picture the steps by which the Fern sporophyte may have arisen 

 by changes in individuals homologous with the original sexual form. The pro- 

 thallus, especially in the Ferns, must have departed much less widely from the 

 ancestral Algal form than the sporophyte ; this may be connected partly with the 

 conditions to which it remains adapted, and partly with the fact of its growth 

 being in nature cut short by the early formation of the embryo upon it. The 

 various cases of apogamy which have been observed form an almost complete 

 series of transitions between prothallus and sporophyte, and have been used to 

 frame a provisional hypothesis of how the alternation in the Ferns might have 

 arisen, if it did not come about in the way suggested by the antithetic theory. 



All such use of the facts of apogamy and apospory is liable to the criticism that 

 they are teratological in their nature, and are not a safe guide in a morphological 

 question of this sort. There are many facts which go far to justify such a view, 

 but we should, I venture to think, be unwise to leave the consideration of these 

 phenomena altogether on one side. Not only can no sharp line be drawn between 

 variations (the use of which in evolutionary questions none will deny) and mon- 

 strosities, but, apart from the particular organic forms which result, we appear to 

 be dealing with a capability of many — perhaps all — Fern prothalli to assume 

 characters of the sporophyte ; a general property of the gametophyte of this kind 

 cannot be disregarded. A fuller knowledge than we possess of the causes of apo- 

 gamy is, however, necessary before the bearing of the phenomenon on the nature of 

 alternation can be properly estimated ; such knowledge may lead to an explana- 

 tion more in accordance with the antithetic theory than any which has yet been 

 given. 



AVhether the homologous or the antithetic theory is to be considered the more 

 probable has an obvious bearing on morphology. But there is a wide difterence 

 between considering the two generations homologous with one another in the sense 

 that the spore-bearing generation is ultimately to be traced back to modification of 

 the sexual, and the viesv that any special structure of the sporophyte is strictly 

 homologous by descent with any structure in the gametophyte. Special evidence 

 •would be necessary before such a conclusion could be drawn, and, so far as I am 

 aware, no such case has been shown to exist. Not only, then, does the question of 

 the nature of alternation of generations in the Archegoniates appear to be au 

 open one, but there seems no reason to apprehend confusion in comparative mor- 

 phology, whichever of the two theories be adopted as a working hypothesis. 



In concluding this account of some of the main factors in the problem which is 

 the subject of this discussion, three subsidiary questions may be suggested — -the 

 probable line of descent in archegoniate plants, the bearing of the cytological facts 

 on the question, and the significance to be attached to apospory and apogamy. 

 None of these questions, anymore than the general one of the nature of alternation. 



