276 TEAlsSACTIOXS AXD PEOCEEDINGS OF THE [Sess. Lix. 



Such a thing is clearly impossible, and investigation has 

 disclosed a process of reduction of the number to one-half, 

 at a period prior to spoie-formation. It is on this process 

 of reduction, and on various matters connected with it, 

 that I desire more especially to concentrate your attention 

 this evening. 



The first general and authoritative discussion of this 

 matter has been due to Strasburger, before the British 

 Association at Oxford; and his paper, published in the 

 current number of the "Annals of Botany," will un- 

 doubtedly give a strong impulse to the study of this 

 question. From various sources evidence has lately been 

 accumulated that, as regards the nuclear condition, there is 

 a difference between those two generations which alternate 

 with one another so conspicuously in the life-history of 

 archegoniate plants. Every elementary student knows 

 that the life-cycle of a fern includes two phases, differing 

 greatly in external form, — the fern plant, or neutral 

 generation, or sporophyte ; and the prothallus, or sexual 

 generation, or gamdophyte. A more advanced student will 

 tell you that, on grounds of comparison, we may hold that 

 the sexual generation was, in the progress of descent, the 

 original one, and that the neutral generation or sporophyte 

 arose as a derivative of the former, — a phase intercalated 

 in the course of descent ; that it was apparently not pro- 

 duced by a mere transformation of the sexual plant, but 

 arose as a new growth by elaboration of the product of 

 sexuality, — the zygote. Theorists, amongst whom I must 

 class myself in this matter, have attempted to connect 

 the origin of this wonderful development with the action 

 of external causes. I believe myself that the expansion, 

 though perhaps not the origin, of this most striking pheno- 

 menon of antithetic alternation is to be closely correlated 

 with the migration of li%dng organisms from an aquatic 

 habitat to exposed land-surfaces. But whatever views we 

 may hold as to the influences which brought it about, and 

 whatever our morphological opinions based on comparison, 

 we who are specially interested in alternation must feel 

 that a crisis has now arrived in the progress of our study ; 

 for facts are being rapidly disclosed which show that the 

 distinction of the alternating generations is to be based not 



