158 Isokontx 



non-motile existence such as predominates in the Palmellacese. In the latter 

 family the cells sometimes escape from their old walls, develop a pair of 

 cilia, and become motile zoogonidia. Blackman remarks that the ' formation 

 of zoospores is then nothing but reversion to an ancestral type of vegetative 

 existence for a biological advantage, and all the vegetative existence of the 

 higher Alga? is phylogenetically a new intercalation into the life-history of 

 the motile Chlamydomonad which is permanently in the zoospore-condition, 

 though walled, and in which zoospore-formation and vegetative cell-division 

 are one and indistinguishably the same thing.' 



There seems to be no doubt that Chlamydomonas has itself arisen from 

 certain of the autotrophic Flagellata, through the family Polyblepharidaceae. 

 In this family intermediate forms are found which, although exhibiting 

 preponderating algal characters, yet afford clear evidence that it is quite 

 impossible to draw a sharp line of demarcation between the Flagellata and 

 the primitive Green Algae. Zumstein ('99) has shown that some Flagellates 

 may be either green and holophytic or colourless and saprophytic according 

 to the available food-substances. Just as green and brown organisms occur 

 among the Flagellata, so do colourless forms occur among the lower Algas. 

 Polytoma avella Ehrenb. is really a colourless member of the Chlamydo- 

 monadese; and Prototheca moriformis Kriiger, Mycotetraedron cellare Hansg., 

 and Chionaster nivalis (Bohlin) Wille should be similarly regarded as colour- 

 less members of the Autosporacese. 



The first of the three lines of descent along which the Protococcales 

 appear to have evolved, in consequence of the Volvocine tendency, has 

 resulted in the production of a series of organisms of gradually increasing 

 complexity, which constitute the Volvocacea?. This line of evolution has 

 terminated blindly in the genus Volvox. 



The second line of evolution, resulting from the Tetrasporine tendency, 

 is the most important, having been the chief line of descent of the great 

 majority of the Isokonta?, including nearly all the higher types of the Green 

 Alga?. Indeed, it is along this line that in all probability all the higher 

 green plants have arisen. In its initial stages this tendency resulted in 

 the production of a series of forms in which vegetative cell-division gradually 

 replaced the formation of zoogonidia as the chief method of propagation. In 

 this way the Palmellacese came into existence, and from this family most of 

 the other Isokontan families have arisen. 



The third line of descent from the Chlamydomonad type is to be asso- 

 ciated with the Chlorococcine tendency\ which has resulted in various Algaj 

 from which vegetative cell-division is absent, multiplication of individuals 

 taking place only by the formation of zoogonidia or motile gametes. The 

 plants are mostly coenocytes and unseptate, with a gregarious habit. This 

 1 Previously known as the Endosphaerine tendency. 



