324 SCIENCE PROGRESS. 



though he objects to the terms used by Celakovsky and 

 employs instead embryonal alternation, distinguishing the 

 generations as proembryonal and embryonal respectively. 

 Antithetic alternation is considered to be peculiar to plants, 

 a close analogy to alternation of generations in animals 

 being found in alternation of shoots. The paper is of 

 special interest from the broad philosophical standpoint 

 assumed, which leads to the discussion of several factors 

 in the problem, which have frequently been lost sight of. 

 Thus Braun lays stress on the possibility that the develop- 

 ment of a group of organisms may have been monophyletic 

 or polyphyletic and gives point to the general discussion by 

 suggesting that the Mosses are to be considered " as a 

 further and, as it appears, geologically late development 

 from Thallophytes." In discussing the phylogenetic signifi- 

 cance of abnormalities, he points out that it is in most cases 

 incorrect to assume that they are atavisms ; they are rather, 

 of the nature of " morphogenetic possibilities ". Lastly, 

 this important paper deserves notice for the picture it 

 presents of the state of " Unsickerheit unci Verwirrung" in 

 which the question of the nature of alternation had re- 

 mained during the twenty years after the publication of the 

 Vergleichende Untersuchungen. 



In the year preceding this paper, the first case of the 

 direct vegetative production of the fern plant from the 

 sexual generation (apogamy) had been recorded, 1 and in 

 1876, Pringsheim, 2 after repeated trials, induced the corre- 

 sponding phenomenon 3 of the vegetative production of the 

 sexual from the tissues of the asexual generation in Mosses. 

 These discoveries, the nature of which will be referred to 

 later, led him to a theoretical view of the nature of alterna- 

 tion in archegoniate plants, which is essentially different 

 from the antithetic theory. In the following year, 4 he de- 

 veloped this theory in detail. Two types of alternation in 

 plants are recognised, (1) the vegetative alternation of shoots 

 exhibited by both sexual and asexual generations, and (2) 



1 Farlow (1). 2 p r i ngs h e im (4). 



3 This was afterwards termed " Apospory " by Vines (1). 



4 Pringsheim (5). 



