252 REPRODUCTION, EVOLUTION, ETC. 



Monostrofna represents another intermediate condition in which the 

 enlargement of the zygote can be regarded as an incipient delay 

 before the reduction division takes place. 



Summary and Conclusions 



In fig. 1 66 are set out some simplified diagrams of the fife cycles 

 of the principal algal types to be found in all three groups. They have 

 all been drawn up on the same principle so that comparisons will 

 be rendered easier. On the hypothesis that the Chlorophyceae are 

 probablv the original ancestors of most of the algal groups, the 

 types of life cycle to be found there have been made the basis of the 

 other diagrams. Chlamydomonas, Ulothrix and Coleochaete can all 

 be regarded as simple types in so far as their life cycles are con- 

 cerned, although it is conceivable that the life cycle of Coleochaete 

 may have been secondarily reduced to the wholly haploid stage, or, 

 more probably, that morphological evolution took place without 

 any comparable change in the life history. From a morphological 

 and reproductive standpoint Coleochaete would appear to be the 

 only member of the Chlorophyceae from which the Eu-florideae 

 might be evolved directly, and it is worth noting that the life cycles 

 of Coleochaete and the primitive Eu-florideae, Scinaia, Nemalion, 

 Batrachospermum, are identical. It is true that there are differences 

 in structure between Coleochaete and the primitive red algae, but so 

 long as there is a complete lack of any intermediate stages it is not 

 necessarily justifiable to abandon such an origin because there is 

 an equal lack of intermediate stages for any other source of the 

 Rhodophyceae (cf. p. 256). It would seem, therefore, that a study 

 of the life cycles of the Chlorophyceae and Rhodophyceae can lead 

 one to two conclusions : 



(i) Their phylogenetic history follows parallel lines whereby 

 they commence with a wholly haploid generation and the diploid 

 generation is subsequently interpolated through a delay in the 

 occurrence of meiosis. Svedelius (1931) has suggested that the 

 delay in meiosis came about gradually, but cytologically it is 

 perhaps easier to imagine one or more sudden delays resulting in 

 two morphologically similar generations, one of which, the diploid, 

 subsequently may have undergone modification. 



(2) There are grounds for believing that some of the filamentous 



