The Origin of Species 331 



cell and sperm is effected much in the same way, 

 but the history of the zygote in animals and plants 

 is, as a rule, not at all alike. In the animals the 

 egg develops at once into the embryo, which sooner 

 or later, either directly or after a metamorphosis, 

 becomes a single individual like the parent. This 

 is rarely the case in plants. In the green algse, the 

 zygote usually becomes a resting spore, whose main 

 function is to carry the plant over periods of stress. 

 Only rarely, and this is especially the case in such 

 marine types as Fucus, does the zygote develop at 

 once into the definite plant. In the majority of 

 the fresh-water algae, as we have seen, the develop- 

 ment proceeds only after a period of rest, and 

 though the zygote may germinate directly into a 

 new plant, much more commonly it first divides into 

 a number of free cells, each of which gives rise 

 to a new individual, and this interpolation of a neu- 

 tral stage between the zygote and the production 

 of new sexual plants, becomes more pronounced in 

 the higher plants. 



While the alternation of generations, which is 

 so conspicuous in all green plants above the algae, 

 is sometimes met with in animals, as in some 

 insects and hydroids, it is far less common than 

 in plants. In all of the higher plants, from the 

 ferns to the seed-plants, it must be remembered 

 that the predominant phase is the non-sexual 

 sporophyte. Much confusion has arisen from over- 

 looking this fact. The sexual cells of a flowering 



