THE NATURE OF ALTERNATION, ETC. 321 



of alternation of generations which expresses itself in the 

 fruit-formation. " 



At this time descent was not a recognised factor in 

 morphological comparison. In reading the clear statements 

 of fact and the definite conclusions arrived at on comparative 

 grounds in the works of Prino-sheim and Hofmeister this cir- 

 cumstance is readily forgotten. It applies however to papers 

 published for some years after the first edition of the Origin 

 of Species. The importance of descent in such morphological 

 questions was recognised and insisted upon by Haeckel in 

 1866. 1 He discussed alternation of generations in plants 

 and animals and, though some of his comparisons were mis- 

 leading, the main conclusions so far as they relate to plants 

 must be mentioned on account of their bearing on later 

 theories. In Bryophyta and Pteridophyta, which he grouped 

 together as Prothallophyta, Haeckel recognised true alterna- 

 tion (Metagenesis), but he compared the protonema with 

 the fern prothallus, and the leafy moss plant together with 

 the sporogonium to the fern-plant. Except for isolated 

 cases of reproduction by separable bulbils he did not con- 

 sider that true alternation was exhibited by Phanerogams. 

 In the latter the succession of vegetative and reproductive 

 shoots was distinguished from Metagenesis under the name 

 of Strophogenesis or succession of generations. 



The influence of Haeckel's views on alternation is clearly 

 traceable in a paper by Strasburger, 2 which appeared some 

 years later ; the conclusions are of interest from a historical 

 point of view, although they are not maintained in later 

 works by this author. He distinguished as true alternation 

 of generations (Metagenesis) the succession in the life 

 history of two or more genealogical individuals which have 

 become more or less unlike. This is recognised as existing 

 in the vegetable kingdom only among Thallophytes. From 

 the Mosses onwards the alternation is considered to be of a 

 different nature (Strophogenesis), and to have arisen by 

 the splitting of the genealogical individuals of a single 

 generation into physiological individuals. Misled by the 



1 Haeckel. 2 Strasburger (1). 



