62 SEX AND HEREDITY 



normal relations between the protozoon and the sur- 

 rounding world whether the actual change takes place 

 in the circumstances of the outer world or in the vital 

 activities of the creature itself. 



If we turn from the Protozoa to the more complicated 

 types of animal we find processes taking place which are 

 in their essence the same as those which we have studied 

 in the Protozoa. Any one of these more complicated 

 animals say a Lobster, a Fowl, or a Man begins its 

 existence as a single cell a zygote formed by the fusion 

 together of two gametes. Then there follows a process 

 of fission repeated over and over again the zygote 

 dividing into two cells, each of these dividing again and 

 again, and so on for hundreds or thousands of generations. 



But there is this striking difference from the Protozoon. 

 In the latter, when the zygote divided into two, the two 

 cells so formed separated, swam away, and lived their 

 own lives as independent individuals. In the animals 

 higher in the scale however, the cells remain attached 

 together so that we get a coherent mass of cells, 2, 4, 8, 

 16, 32, 64, 128, and so on. This mass of cells becomes 

 larger and larger with successive cell-divisions it grows 

 it is the body of the individual it goes on growing 

 until at last cell-division slackens off and the animal 

 attains a more or less definite adult size. 



Just as a large community of civilized human beings, 

 such as a great city, requires an enormously complicated 

 organization to provide for its various needs, as compared 

 with a savage community composed of a few almost 

 independent individuals, so this immense community of 

 cells which constitutes the body of one of the higher 

 animals has to undergo an extraordinarily complicated 

 process of organization. Certain tracts within its living 



