VARIATION. . 35 



mum which still allows of development. In this way we can 

 study the effect of atmospheric pressure, and the action of 

 light. But we know for certain that the inherited factors in the 

 development of the organisms cannot be so regulated. They 

 are either present in the germ, or they are absent from it. This 

 has the result that only such transmittable developmental fac- 

 tors can be studied in their action as are not absolutely indis- 

 pensable for at least a partial development. There is still an- 

 other circumstance which hinders a study of this class of devel- 

 opmental factors. Such a factor can only be studied by a com- 

 parison of individuals, of which some have developed under 

 the influence of it, and some without its cooperation. And as 

 yet we have no means of eliminating these genetic factors. If in 

 a species of plants or animals, all the individuals and all the 

 germs produced contain a certain developmental factor, we 

 can never study the action of this factor on the development, 

 as the individuals which might have developed without it are 

 not available for comparison. 



We saw, that the different environmental factors which in- 

 fluence the development, can sometimes be different in inten- 

 sity. In direct proportion to the variation of intensity in which 

 a certain factor contributes to the development. The result, and 

 therefore the final qualities of the organism may vary. Other 

 developmental factors remaining constant, the amount of 

 water available for growth may be directly related to a certain 

 character of a population of plants, for instance to weight or 

 height. The plants which have received more moisture may be 

 taller than those which have had less water, and it is conceiv- 

 able that in a field in which water is irregularly distributed, the 

 plants are of varying height and all gradations of height may 

 be present between that of the lowest and that of the tallest 

 individual. Another example is heat. (Fig. 3). If we sow a num- 

 ber of seeds of the same pure strain, each in a little pot, and 

 we put these pots in a row in a cold environment, then place a 

 source of heat at one end of the row, we will see that the plants 

 nearest this point are tallest, and those furthest away from it 



