THE LAWS OF HEREDITY 



passes on the substance of an organism to its 

 descendants in so definite and tangible a way that 

 there is nothing mysterious at all about the ob- 

 served fact that children are closely comparable 

 to the parent. It is inconceivable that they should 

 be otherwise under the circumstances. The basal 

 fact of heredity, then, observed thus as it were at 

 its source, seems not in the least mysterious, but 

 a mere matter of fact. 



When we reflect that complex higher organisms 

 are made up of groups of cells aggregated and 

 differentiated to perform specialized functions, 

 and that all growth takes place through cell divi- 

 sion, it will be clear that the distinction between 

 the single-celled organism and the complex higher 

 organism is not as radical as might at first thought 

 appear. Each individual even of the highest 

 forms of life, including man himself, begins ex- 

 istence as a single cell and grows and develops 

 only through the countless redivisions of that 

 cell. 



There is, however, a highly important modifi- 

 cation to be noted in the fact that the cells of the 

 higher organism become differentiated into di- 

 vergent groups, capable of performing different 

 functions. And, from our present standpoint, the 

 thing to be particularly noted is that the tissue 

 serving the purpose of reproduction of the species 

 is segregated; and that this segregation takes 

 place, in the case of higher animals, at a very 

 early stage of the embryonic development of the 

 individual. 



[263] 



