374 THE BIOLOGY OF BIRDS 



Let us suppose that in a pair of homologous chromosomes, 

 one (A) has a factor (x) for a particular kind of iris pigmenta- 

 tion, which the other (a) has not. When the first polar 

 body is formed, it will take with it either A or a. If it takes 

 A and therefore x, and if the ripe egg is fertilised by a sperma- 

 tozoon also without x, the resulting offspring cannot have the 

 particular kind of iris pigmentation for which x was the 

 germinal representative. In another egg the polar body may 

 remove the chromosome a. 



If we compare the inheritance to a pack of cards, each 

 card representing a chromosome, there is a throwing away 

 of half the pack in the maturation of the egg. There is a 

 corresponding reduction in the history of the sperm-cells, 

 which are divided into two sets, each set with a different 

 half-pack. In fertilisation the egg's half-pack and the 

 sperm's half-pack form a complete pack once more. 



The facts are intricate, but the general idea is clear that 

 in the history of the germ-cells, there are various oppor- 

 tunities for shuffling the cards — for permutations and com- 

 binations among the genes. Among these opportunities we 

 notice the chance of " crossing over," the reduction-divisions 

 of maturation, and the re-combining in fertilisation. 



But what can one say of qualitative variations which 

 suggest something more than " shuffling " ? What can one 

 say as to the origin of the qualitatively new ? Perhaps the 

 less said the better, since the answers are almost wholly 

 speculative. What might induce changes in the genes 

 themselves or what might lead to the origin of a new gene ? 

 The living germ-cell is a potential organism ; it has 

 withm it the gains of the ages ; it is a treasure-house : 

 perhaps like other living things it has an intrinsic capacity 

 for re-arranging and re-organising itself. Perhaps the 

 trigger of its variability may be pulled by deeply saturating 

 environmental changes, such as the setting in of an ice age. 

 Perhaps the complexity of the germ-cell may be disturbed 

 and re-arranged by changes in the nutritive streams of the 

 blood and lymph. But the origin of the distinctively new 

 remains the most difficult problem in biology. 



