96 GENETIC VARIATIONS 



division of the parental cell, each of the two new cells pro- 

 duced transforming into an adult individual. Thus the one- 

 cell organisms show us the nature of heredity and variation 

 as these occur when there is but a single cell division between 

 parent and offspring. 



Yet it is important to realize that even in this reproduction 

 by a single division, the individuals of the new generation are 

 produced by development. Even the unicellular organisms 

 are complex systems of structures; they are small machines. 

 We cannot multiply machines merely by cutting them in 

 two; try that on a typewriter or a gasoline engine! If the 

 organism is simply divided, the unity and harmony of its 

 machine-like structures are lost. During division many or 

 most of the definite organs disappear, by absorption or disso- 

 lution, and are then after division produced anew; they are 

 not transferred bodily from parent to offspring. This is true 

 of all the unicellular organisms, even the simplest, but it is 

 best seen in the more complex types. Some of the infusoria 

 bear on their bodies numerous differentiated appendages 

 serving as organs of locomotion; these are arranged on the 

 body in a definite pattern. When the animal divides, these 

 appendages disappear, and as the body separates into two 

 parts a new set of appendages appears and develops separately 

 on each half, finally arranging themselves in the typical 

 pattern (Figure 15). Thus inheritance of the organs and 

 pattern is not direct transfer from parent to offspring, but is 

 new production of structures like those which the parent pos- 

 sessed. Yet even so, reproduction by a single cell division is 

 relatively simple in comparison with the long process of 

 development with many successive cell divisions, that occurs 

 in higher organisms. To this difference is perhaps to be 

 attributed some of the peculiarities of heredity and variation 

 in these organisms. 



