May 1900.] IJOTANICAL SOCIETY OF EDINBURGH 299 



the converse of wliat is believed to be the development uf 

 the race. 



To explain tliese cases of inversion of the order of 

 sequence, where there is a want of correspondence between 

 ontogeny and phylogeny, Mehnert formulates his principle 

 of " time displacements." He would correlate these 

 inversions with changes in the structural complexity and 

 functional importance of organs. 



He submits that the rate of development of an organ 



is accelerated in proportion as its structural complexity 



and its functional importance increase, while with 



diminution in physiological importance and structural 



differentiation, the rate of development is in the same 



degree retarded. Mehnert adduces numerous examples 



and measurements from the animal kingdom in illustration 



of his principle. 



[A lucid account of Melinert's theory, and a discussion of it in so f;vi- 

 as it is of interest to the biologist, will be found in the '" Proc. of the 

 Scots. Micr. Soc." for 1898-99, by Prof. J. Arthur Thomson. To this 

 paper I am indebted for an introduction to Mehnert's principles, and 

 from it I take the term ''time displacements.''] 



My object is to point out the interest of this principle 

 from the botanist's point of view, and to cite illustrations 

 in support of it, in the development of the sporophyte. 

 In addition, its bearing on the development of tlie game- 

 tophyte of the higher plants will be indicated, and other 

 cognate matters of interest alluded to. 



Botanists now generally admit the existence of an 

 alternation of generations, at least in the higlier plants : 

 the life cycle consists of a sexual and an asexual phase, 

 which follow each other in regular succession. This 

 alternation is seen to greatest advantage in the Vascular 

 Cryptogams ; is obvious in the mosses ; is present, though 

 not so conspicuously, in the flowering plants ; and is only 

 just indicated in the Thallophytcs. 



While the fact of the existence of such an alternation 

 is accepted, phylogenetic relationship between the two 

 generations is disputed. Various theories of the sporo- 

 phyte have been advanced, among others recently by 

 Lang (5) and Klebs (9). Perhaps the two most discussed,, 

 however, are those of Pringsheim (3) and Bower. In 

 that of the former, the two generations are looked upon 



