VARIATION. 45 



fat, water, salt, we need such things as the heat of the oven, 

 and we need the skill of the baker. If any one of these important 

 factors is lacking, bread cannot be produced. If there is no 

 water, there will be no bread. If the baker is drunk, the result 

 will be the same. It cannot be said that any of these necessary 

 links in the chain of factors is "the determining factor." All 

 the other factors being given, the last link completes the chain, 

 and therefore in that particular instance can be said to deter- 

 mine the result. 



The production of such a complex organ as a chicken's tail 

 must be the result of a development by the interaction of quite 

 a number of different factors, quite as diverse as the lot of 

 factors in the production of bread. But some, on seeing that 

 the difference between a normal and a rumpless fowl is some- 

 times due to the presence or absence of one single gene, will 

 look upon this gene as upon "the determiner" for the tail. It is 

 clear, that if we could succeed in getting hold of this particular 

 gene, and putting it into the germplasm of some other animal 

 without a tail, such as a hedgehog, or a cavy , or a sea-urchin, we 

 could not very well hope to see it develop a chicken's tail. 



If in our example of the bakery, eight different factors are 

 absolutely indispensable for the production of bread, each of 

 these eight things may in its turn be the only missing link, and 

 as such "determine" bread or its absence. But no amount of 

 this "determiner" introduced into any other combination of 

 things excepting into this combination of the other seven, will 

 determine bread. No matter how much water one pumps into 

 any ordinary building, it will not produce one loaf of bread. 



We cannot rightly say, that either the lacking factor or the 

 result are "latent." If the flour runs out, none of the customers 

 of the bakery will at breakfast- time take very kindly to the 

 assertion that the bread is there all the same, only in a latent 

 condition, because excepting flour all the ingredients are ready. 

 And the statement that an albino mouse which, when mated 

 to blacks, will produce agouti young, carries the latent 

 factor for agouti colour, or carries latent agouti will not be 



