Studies on variation and selection. jco 



all, or only if they happen to be together with certain combinations 

 of other factors. 



A very important proportion of genes have less influence on the 

 development of some organisms, than other, non-genetic factors under 

 the influence of vvliich the same organisms develop. This being so, 

 continuous variation is not necessarily a function of non-genetic factors 

 (the environment in the widest sence) exclusively. In other words, 

 continuous variation may be wholly non-inheritable, but it may also 

 be partially or wholly inheritable, and inheritable variation may be 

 continuous as well as discontinuous. 



Now we come to the second part of the question. Is disconti- 

 nuous variation always caused by presence and absence of genetic 

 factors ? Numerous instances of discontinuous variation have been 

 studied, which can be readily explained by presence and absence of 

 fewer or more genetic factors. But on the other hand, just as 

 genetic factors mav have more or less influence on different organisms, 

 thus helping one time to pioduce a continuous, another time a dis- 

 continuous variation, so may the influence of non-genetic factors be 

 more or less important. Most influences of an external origin, most 

 non-genetic factors, do themselves vary in intensity. And in so far 

 as they vary themselves, varies the influence they exert over the 

 development of the plant or animal. Thus, the height of certain 

 plants may vary in direct proportion to the amount of moisture they 

 receive. But not all external influences are of such a nature. Some 

 non-genetic factors vary continuously, it is true, but in their action 

 upon an organism there may be critical points, at which this changes 

 abruptly. Take for instance temperature in its influence on plant- 

 growth. Its influence may, within certain limits, be proportional to 

 its intensity, but there is, in its action upon certain plants, at least 

 one critical point, at which this influence changes abruptly. This is 

 the freezing-point of the plant's sap. 



The discontinuous variation brought about by non-genetic deve- 

 lopmental factors, will very often be due to a physiological reaction 

 of the organism, which changes at a critical point, translating itself 

 in a marked morphological difference. The mechanism of this may 

 or may not be well understood, the facts are clear enough. 



Certain mammals, when moulting in a high temperature, produce 

 pigmented fur, but when moulting when the temper.iture is low, their 

 new hair is colourless. The mechanism of this change is fairly well 

 understood. For animals, moulting when the temperature is changing, 



