104 HEREDITY AND VARIATION 



tetrad, and there are 16 possible combinations — viz. a, B, C, D ; 

 A, b, C, D ; A, B, c, D ; A, B, C, d ; a, b, C, D ; a, B, c, D ; a, B, C, d ; 



a, b, c, d ; and eight others which may be got by replacing small 

 letters by capital letters and vice zersd. The number of possibly 

 different offspring would be 16 2 . 



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

 suggesting the possibilities of variation. 



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 liber- 

 ated to become an offspring is not likely to be identical with the 

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

 ing 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 ? 



