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 tersd. The number of possibly 

 different offspring would be i6 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- 

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



i. 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-pell f rprn which jt js descended by continuous cell-division ? 



