BIRDS AND EVOLUTION 373 



§ 5. Possible Origins of Variations 



It is not difficult to understand how certain kinds of 

 variations may arise, namely in the course of the life-history 

 of the germ-cells which includes several notable oppor- 

 tunities for permutations and combinations. 



In the case of many heritable characters it seems certain 

 that they are represented in the egg-cell or the sperm-cell 

 by material items of some sort, usually called " factors " 

 or " genes." These are the initiatives in the germ-cell 

 which develop into the characters of the adult, after the usual 

 trafficking with the appropriate " nurture." In some cases 

 it is certain that these " factors " lie in a longitudinal series 

 in the nuclear rods or chromosomes of the nucleus of the 

 germ-cell. By observations, as patient as they are ingenious, 

 it has been found possible in certain cases to map out the 

 chromosome, and to say " the factor for eye colour lies 

 about the centre of the third chromosome " ! 



Now in the history of the egg-cell there is a process of 

 nuclear maturation, during which half of the chromosomes 

 are expelled — forming what is called the first polar body, 

 which comes to nothing. The number of chromosomes in 

 the nucleus of the egg-cell is reduced to half the normal 

 number, and the same reducing (or meiotic) division occurs 

 in the history of the sperm-cells, so that the ripe sperm-cell 

 has also half the normal number of chromosomes. In 



fertilisation the normal number is restored : -+-=«. 



2 2 



Returning to the ripening egg -cell, we must notice that 



the chromosomes lie in what are called " homologous pairs," 



one of which is believed by many to be, to begin with, of 



paternal origin, and the other of maternal origin. In any 



case they contain similar sets of factors, and in their close 



approximation there is sometimes a strange " crossing over," 



a block of the one chromosome exchanging with a block 



from the other — which might of itself lead to some new 



permutation if the corresponding blocks are not quite 



identical. 



