EVOLUTION 263 



suggested that they are derived from the Volvocales on the basis of 

 a resemblance between them and the simpler forms, e.g. Gongrosira. 

 It is, however, probably more correct to interpret these simpler 

 forms as reduced rather than primitive. 



From the above two schemas it will be seen that there are a 

 number of definite morphological tendencies, and it has already 

 been pointed out that these various lines of morphological de- 

 velopment are repeated in the different algal groups. Table IV from 

 Fritsch (1935) provides examples of parallelism in evolution among 

 the simpler types of algae. 



Apart from these examples of evolution among the simpler algae 

 it is also found that other evolutionary tendencies can be observed 

 among the more advanced types of algae. Such tendencies are 

 illustrated in Table V. 



The concept of the heterotrichous habit was first advanced by 

 Fritsch in 1929, and it has become increasingly evident that an 

 understanding of this habit is of fundamental importance in con- 

 sidering any phylogenetic or evolutionary problem among the 

 algae. In the primitive state both the prostrate and erect systems 

 must be present, but during the course of evolution one of these 

 has frequently become reduced or lost, e.g. in Endoderma (Chloro- 

 phyceae), Strehlonema (Phaeophyceae) and Melobesia (Rhodo- 

 phyceae) only the prostrate system remains. In contrast to this 

 the thallus in Draparnaldia and most of the Florideae represents the 

 erect system, the prostrate system having been reduced or lost. 

 Another fact in connexion with this phenomenon, which needs to be 

 re-emphasized, is that the most advanced Chlorophyceae exhibit the 

 heterotrichous habit in its primitive state, whilst this condition is 

 only found fully developed among the simpler Rhodophyceae and 

 Phaeophyceae since in the more evolved types one or other of the 

 systems is reduced. The possible implications of this observation 

 are immediately obvious. 



Finally, a word may "be said about the time when the diflferent 

 groups first made their appearance. Most authors would consider 

 that the Cyanophyceae and Chlorophyceae are the most primitive 

 and therefore appeared first. If, however, the Rhodophyceae and 

 Phaeophyceae have a flagellate origin then all four groups may be of 

 almost the same antiquity. There are some workers who believe 

 that the Cyanophyceae are the most primitive group and that they 



