The Recapitulation Theory. 83 



which appears to be well justified by the facts. The life history of 

 Griffithsia is in general parallel to that of Polysiphonia, the tetra- 

 sporic plants apparently having 14 chromosomes and sexual plants 

 7, though there is some doubt about the accuracy of these counts. 



According to the views of Lewis, there is (1) an antithetic 

 alternation between sexual plants, representing the gametophytes, 

 and the sporogenous cells of the cystocarp, representing the sporo- 

 phyte, (2) a regular succession of tetrasporic and sexual individuals 

 representing an homologous alternation of generations but not 

 equivalent to the alternation in Archegoniates. He believes that 

 the tetrasporic plant has been intercalated in the life history, not 

 through the "gradual differentiation of a simple product of the 

 germination of the zygote," which is the pith of the antithetic 

 theory ; but, as Yamanouchi suggested, through the suppression of 

 chromosome reduction in the formation of a carpospore which then 

 at once grew into a tetrasporic plant. 



In concluding a brief survey of this very large subject of alter- 

 nation, we may remark that the antithetic theory, which continues 

 to receive the larger number of adherents, implies just such a 

 lengthening of the life-cycle as is indicated by recapitulatory 

 phenomena. The homologous theory, on the other hand, would 

 account for the alternation through the sudden intercalation of a 

 generation in Algae as the result of a change in spore development 

 which is essentially mutational. In Archegoniates, the homologous 

 view would apparently imply the gradual simultaneous progressive 

 differentiation of gametophyte and sporophyte through differences 

 in the environment in which the asexual spore and the fertilized 

 egg develop. But as Farmer 1 has pointed out, it is impossible to 

 imagine how the Bryophyte sporogonium at any rate could have 

 arisen through modification of the gametophyte. It seems clear 

 that this structure at least must represent an intercalated phase 

 progressively developed. Each group of plants must then, as 

 Farmer has emphasized, be separately considered on its own merits, 

 on the basis of the historical probabilities as determined by 

 comparative morphology. 



Recapitulation phenomena in Gametophytes. 



It may be noted in passing that although plants "climbed out of 



the water " at a remote period, yet large groups of Bryophytes and 



Pteridophytes still survive although imperfectly adapted to life on 



dry land. This, as Bower points out, is probably because the 



' NEW PHYTOL. 8; 113, 1909, 



