CHAPTER XII 



ORIGIN OF SEX AND OF THE SOMA. THE 

 GREEN ALG^ 



Life probably began in the sea, and some at least of 

 the earliest organisms were probably minute free- 

 floating forms which were able to absorb and use 

 light-energy to build up their bodies from simple 

 inorganic substances, and must therefore be classed as 

 plants. Of these some developed flagella and became 

 actively free swimming, and their descendants are 

 still represented by yellow, brown or green flagellate 

 unicellular plants (algae) which live in the sea or in 

 fresh water. All these free-floating and free-swimming 

 organisms, whether animals or plants, are collectively 

 called plankton. I One series of green flagellate plankton 

 algae, nearly all of which are confined to freshwater, 

 illustrates very beautifully the way in which the 

 differentiation of sex among conjugatmg cells came 

 into existence, and the same series also illustrates the 

 origin of what is called the soma,^ that is the body 

 of an organism fls opposed to its reproductive cells. 



"Immortality" of Unicellular Organisms. — The body 

 of a unicellular form, such as an amceba, a bacterium 

 or a yeast plant, not only feeds and grows, it also 

 divides (or buds) and produces new individuals of the 

 species. This production of new individuals, or repro- 

 duction as it is called, is in origin simply an extension 



' Greek wAay/ads, wandering. ' amfia, body. 



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