334 Causes and Course of Organic Evolution 



Examination of living plants higher than the Thallophyta, 

 in all of their life phases, and consideration of this in the light 

 of pala^ontological evidence, equally suggest that two probable 

 series of organisms early started from a related green algoid 

 ancestry, and progressed along diverging pathways of ascent. 

 One of these probably originated the classes of the Hepaticse 

 or scale mosses, and the Musci or leafy mosses, also the Lycop- 

 odinese or club mosses and selaginellas. These will be spoken 

 of as the Biciliatoe. The other gave rise to the ferns and water- 

 ferns, the Cycadofilices and Gymnospermia, finally attaining 

 a climax in the higher seed-plants or Angiospermia of our 

 existing flora. These will be spoken of as the Polyciliatoe. 



Statement was made earlier (p. 314) that Coleochcete and 

 AphanochcEte might be viewed as nearest living representatives 

 of organisms that started the above. In comparing them and 

 the liigher organisms that we consider they led up to, a point 

 of great morphological interest claims attention. We refer 

 to the graded evolution of alternating generations of sexual 

 or gametophyte, and of asexual or sporophyte growths, in 

 the life-cycle of each individual, and the number of chromatin 

 masses or chromosomes that respectively characterize the 

 cells of these two generations. 



A more detailed study of this has been made in a previous 

 chapter that deals with sex-evolution. But shortly it may be 

 indicated here that, from the next higher class to the algae, 

 namely the Hepaticae, upward to the liighest plants, the cells 

 of the plant body or gametophyte that culminate in production 

 of sexual cells show, during each cell division, half the number 

 of chromosomes up to the point when the egg and sperm fuse. 

 Since each of these introduces as many chromosomes as were 

 typical of the parent plants that bore them, the fertilized egg 

 that is the climax of the sexual or egg-producing or gameto- 

 I)hyte generation shows double the number that it or the cells 

 that produced it had, up to the moment of fertilization. 



But the fertilized egg on segmentation, while retaining the 

 double number, gradually builds up a new structure that is 

 knowTi as the asexual or spore-producing or sporophyte gener- 



