202 ORIGIN OF SEX AND OF THE SOMA 



produced, the structure of the two being identical. 

 The individual sluggishness of the larger gametes in 

 Pandorina is probably merely a result of their larger 

 size, but since they tend to remain passive and also 

 contribute more cytoplasm to the zygote, we see here 

 the foundations laid of the specific characters the 

 femaleness of the egg in the higher forms. In these 

 the differentiation is carried further and has become 

 fixed the smaller (male) gametes are no longer capable 

 of conjugating with one another. They are short- 

 lived cells, incapable of nourishing themselves, but 

 extremely active and sensitive, their sole function 

 being to get to the egg as quickly as possible and 

 contribute their quota of nuclear material to the 

 zygote. The female gamete has taken over entirely 

 the function of feeding the new individual, and the 

 amount of food that can be stored in it is not limited 

 by the need of motility. This is a much more efficient 

 arrangement for giving the new individual a good 

 start in life than the conjugation of isogametes, both 

 of which are motile and neither of which can con- 

 tribute much food to the zygote. 



The origin of sex is an excellent example of the 

 origin of a differentiation which is at first, as it were, 

 accidental, i.e. appearing without reference to its ulti- 

 mate use, but is later fixed and further developed into 

 an extremely efficient working mechanism. The more 

 we learn of the evolution of structure and function in 

 organisms the more we find that something like this 

 is the history of the evolution of new characters. 



In the higher forms the^egg is often, as we shall see 

 in the sequel, much reduced in size because parts of 

 the parent organism take over from the egg the 

 function of providing food for the new individual 

 produced from the zygote. 



