266 Causes and Course of Organic Evolution 



as that wliich suggests and presents evolving sexuality. But 

 both are equally graded, instructive, and fundamental, though 

 they must early have diverged along two pathways. One of 

 these — the Conjugatae — has ended in an organic cul-de-sac, 

 while the other or Zoogametse has led to the development of 

 higher and more complex algse, in turn also has given rise to 

 the archegoniate plants. The latter in fact has become the 

 dominant line, as generally accepted during the past quarter 

 century. 



We can only refer now to the thread-shaped Conjugatae, 

 in which varying degrees of cell affinity are shown in the con- 

 jugating cells. The cells throughout the greater part of the 

 year resemble each other, and are purely vegetative. But 

 at surprisingly definite periods the cells become denser and 

 richer in reserve food. Then in Mougeotia either adjoining 

 pieces of sister cells become cut off (M, uleana) and the com- 

 mon partition between these breaks down ^dth subsequent 

 fusion of contents, or from adjacent cells of an individual 

 thread two hump-like tubes are formed (M. mirabilis), which 

 join and unite their contents into a fertilized conjugate cell. 



But in Debarya and Staurospermum opposite cells of distinct 

 filaments put out tubes which meet, the contents of each cell 

 seem to be equally attracted to the other, and so, after absorp- 

 tion of the bridge partition, fusion of the contents occurs in 

 the middle of the bridge. But in some species of Spirogyra 

 and Zygnema the tubes that meet from opposite cells are rather 

 irregularly directed, though the contents of one cell, or possibly 

 its nucleus, so react on the other that the latter mass flows 

 over to fuse with that opposite. So here we can now say 

 that one cell is a giving or male, and the other is a receiving or 

 female cell. 



But, in the more modified species of Spirogyra already re- 

 ferred to (p. 257), distinct lines of stereoenergizing action 

 are manifested between the opposite or sub-opposite cell nuclei, 

 so that the molecules of each wall are molded into shape, and 

 so placed that they form a tube, each of which exactly grows 

 toward and joins its neighbor, with the result that the con- 

 joint tube thus formed is in exact line with the cell nuclei. 

 This action and reaction relation may be due to the giving 

 off from each of a definite chemical substance, that starts 

 chemotactic action and reaction; or to definite interaction 

 of opposite kinds of energy, much as we might regard a -1- 



