A LINE OF EVOLUTION 57 



like thallus. Thinking in this way, we have little trouble 

 in seeing how several forms of thallus might diverge 

 from the Pleurococcid cell. We must not be forgetful 

 of the fact that Nature sometimes produces startUng 

 mutations, and makes curious jumps; there is a ten- 

 dency to " sport," to kick over the traces of ancestral 

 limitations, and to adventure in new directions. A 

 million Pleurococcid cells might behave according to 

 ancestral traditions, and their progeny might, through 

 an aeon, adhere most rigorously to the old ways; but a 

 half-dozen cells might become restive under a mysterious 

 inexplicable stimulus, and start a new line of evolution. 



From a simple filamentous thallus, arising in the 

 manner just suggested, we can follow later develop- 

 ments. At first almost any of the cells would produce 

 motile zoospores, by a partial reversion to ancestral 

 type, or the filament itself might break up into its com- 

 ponent cells, setting them free to develop new threads 

 by simple division. Later would follow the production 

 of equal gametes, which we may regard as speciahzed 

 zoospores, both gametes being set free from their 

 mother-cells, and motile; and, later still, further speciali- 

 zation would lead to formation of passive egg-ceUs and 

 active spermatozoids. It is along some such line as 

 here suggested that plants like Ulothrix and CEdogonium 

 have probably evolved. These types, in their turns, 

 represent intermediate phases leading upwards to the 

 Liverworts and still more advanced land-plants. 



Thus, we have followed a possible line of evolution 

 from the Chlamydomonas type of cell, through the 

 Pleurococcid form, to the Confervoid Algae, and, later, 

 to land-plants. The significance of the facts and sug- 



