40 VARIATION. 



field, and the remaining seed in a fertile well-watered place, we 

 will, after the plants are harvested, still be able to differentiate 

 the plants of the two lots with facility. But if we sow the seed 

 in two adjacent fields of the same quality and in the course of 

 the summer sprinkle a pail of water over one of the plots once, 

 we shall never be able to sort out the plants of the two lots 

 after they are once mixed at harvest. 



With genetic, inherited factors, the case stands in the same 

 way. If we have a lot of plants of which some have a certain 

 inherited factor, necessary for pigmentation, whereas the 

 others lack this, it will be easy at a glance to distinguish the 

 plants of the two groups. If we mix the plants we are able to 

 sort them again. The variation is discontinuous. 



If we have a number of rabbits or mice, of which some are 

 chocolate and some are black, because the latter have an in- 

 herited factor which the chocolates lack, we can still sort out 

 the animals which are with or without this factor from a 

 mixed lot. But the gap between the darkest, fullest-coloured 

 chocolate, and the brownest, lightest-black is not nearly so 

 large as that between a coloured plant or animal and an albino. 



Another genetic factor in rodents, has still less influence on 

 the development, and therefore, on the characters of the indi- 

 viduals. This is the factor which distinguishes fully-coloured 

 from "fade" ones. (Fig. 5). If we compare two groups of ani- 

 mals, one of which has this factor and the other lacks it, we 

 see an appreciable difference, but the variation is discontin- 

 uous only in certain families. In a group of black animals, the 

 possession or lack of this factor makes so little difference, that 

 the darkest, blackest animals without this factor may be ap- 

 preciably blacker than the lightest and rustiest with it. In 

 agouti animals however, the presence or absence of the same 

 factor, appreciably changes the colour of the ventral side. A 

 population of animals, or plants, which contains individuals 

 with, and without, one genetic developmental factor, may, or 

 may not show discontinuous variation. If the influence of the 

 factor on the development is considerable, the gap between 



