I04 HEREDITY AND VARIATION 



Sutton gives the following table, which is of some interest as 

 suggesting the possibilities of variation. 



Normal Number Number of combina- Number of possi- 



number of of Tetrad- tions in the mature bilities in the 



chromosomes groups germ-cells offspring 



8 ... . . 4 . . . . i6 . . . . . . 256 



12 ... . . 6 . . . . 64 . . . . . . 4,096 



16 ... .. 8 .. .. 256 .. .. 65,536 



24 .. ..12 .. .. 4096 .. .. 16,777,216 



Summary. — In certain moods biologists are accustomed to say 

 that they do not know anything in regard to the causes of varia- 

 tion. They imply that it is. of the essence of living creatures to 

 vary, that variability is a primary property of organisms. The 

 sequence of generations is a life stream, changing as it flows. 



In other moods, however, biologists often point out how natural 

 it is that organisms should vary. When the body of the parent 

 is a-making, a lineage of germ-cells is started and the unspecialised 

 descendants of these develop into offspring, which are on the 

 whole like the parent because they are made of the same stuff. 

 " True " twins developed from one ovum are usually almost 

 facsimiles of one another. Why should not the offspring be a 

 facsimile of the parent ? Sometimes, to our eyes, it is quite con- 

 fusable with the parent, but this is not common. Why not ? 



1. It is common to point out that the germ-cell which is 

 liberated to become an offspring is not likely to be identical with 

 the germ-cell which developed into the parent. It has been 

 sojourning in the parent's body, exposed to a variable food 

 stream and often to a variable complex environment, partly 

 somatic and partly external. Is it likely to be exactly the same 

 as the original germ-cell from which it is descended by continuous 

 cell-division ? 



2. It is also to be remembered that if the heritable qualities 

 have their vehicle in the chromosomes, as seems practically 

 certain, then there is during the maturation of the germ-ceUs a 



