BIRDS AND EVOLUTION 385 



progeny test, and rapid progress was secured when those 

 individual birds were sorted out whose offspring proved 

 themselves high-producers. To put it in another way, 

 individual high-laying may be in part modificational ; when 

 it re-appears in the progeny it must be regarded (unless 

 modifications are transmissible) as variational. What the 

 selection did was not to change the constitutional fecundity, 

 but to change the constitution of the poultry population by 

 rejecting those whose progeny were not high-layers. 



Germinal Selection. — There is some interesting evidence 

 in support of the view that there may be a process of selection 

 among germ-cells, so that if they are not alike those with 

 certain congeries of hereditary factors survive while others 

 \vith a different make-up succumb. One of the lines of 

 evidence is curious (C. H. Danforth, 1919). Heterozygous 

 fowls, that is to say with different kinds of germ-cells, were 

 subjected to alcohol vapour. The relative proportions of 

 certain traits, such as brachydactyly, Polydactyly, and white 

 colour, were different from the proportions produced by 

 similar hens in normal conditions. This looks as if the 

 alcohol vapour eliminated certain types of eggs in the ovary. 

 If, then, the survival of germ-cells is determined in abnormal 

 conditions by the genetic or factorial make-up, the suggestion 

 rises whether in normal conditions also the combination of 

 hereditary factors may not have a survival value, which allows 

 some types to be launched on the voyage of development 

 while others are destroyed on the slip. This suggestion of 

 Danforth's appeals to us strongly, for there are various 

 reasons for thinking that the harmony and viability of the 

 future explicit organism is conditioned at the start by the har- 

 mony and viability of the implicit organism — the germ-cell. 



If a toxic agent, such as alcohol, selects between the 

 different kinds of germ-cells, and if this selection be sustained 

 consistently for generations it would tend to result in ortho- 

 genesis, i.e. variation in one direction. Dr. Danforth mated 

 cocks heterozygous in regard to brachydactyly, Polydactyly, 

 colour, and shape of comb with hens homozygous for these 

 features in their recessive forms. A record was made of 



2. c 



