224 THE BIOLOGY OF BIRDS 



their origin is probably in essence like that of any other 

 characters. 



Without straying far from our present path, we may 

 recall some of the possibilities as regards the origin of 

 variations. The opportunities afforded in maturation and 

 fertilisation may bring about a shuffling of the chromosome 

 cards. Fluctuations in the nutritive stream may bring about 

 changes in the germ-cells. External changes may serve as 

 trigger-pulling variational stimuli to the highly complex 

 germ plasm. There is a tendency in matter to complexify, 

 no more explicable than gravitation, but real ; perhaps the 

 living unit utilises this in its germinal experifnenis in self- 

 expression, for that is what we believe mutations, at least, 

 to be. 



Let us suppose, then, that a germ-cell already predisposed 

 to develop into a female-producer was the seat of a variation, 

 say in the determinants or factors corresponding to the future 

 ovary. Let us suppose that this variation was in the direction 

 of producing an accessory yolk-gland. In the course of 

 development the determinant or factor, if consistent with 

 the rest of the constitution, is actualised and there is the 

 beginning of a yolk-gland — an advantageous addition 

 obviously. In the course of time the organism reproduces 

 and its germ-cells have entailed on them (in accordance with 

 the conception of germinal continuity) the determinant or 

 factor of a yolk-gland. But the difficulty immediately arises 

 in the mind that this new hereditary item will be found not 

 only in the germ- cells which will develop into females (where 

 it will be relevant), but also in the germ-cells which will 

 develop into males (where it will be irrelevant). What 

 then will happen to the yolk-gland determinant in those 

 germ-cells that are going to develop into males ? The 

 answer is that it will remain latent, not because its expression 

 would be irrelevant, which is a finalistic idea, but because it 

 arose as a variation in a gamete predisposed to develop into 

 a female. It is solidary with femaleness, which, for us, 

 means a metabolism-ratio or rhythm with relatively pre- 

 ponderant anaboiism. Metaphorically, it is a seed which 



