VARIATION. 



37 



plant or animal which is influenced by it, there may be criti- 

 cal points. To take an instance, let us consider temperature. 

 In the influence of temperature on the growth of plants, there is 

 at least one critical point, the freezing-point of the sap. 



Again, it may happen 

 that a certain non-inherited 

 developmental factor chan- 

 ges in intensity from a 

 minimimi to a maximum 

 and back in long, regular 

 periods, such as the average 

 temperature in the course 

 of a year. Some develop- 

 mental process of an org- 

 anism, and thus some char- 

 acters of that organism 

 may be gone through at 

 two different, regularly re- 



Fig. 4. 

 Discontinuous modification in the 

 leaf -shape of the marsh-buttercup. 



curring periods, which lie far apart, so that the extreme variants 

 at each period do not reach the extremes of the other period. 

 A few examples. The hair of certain animals is coloured only 

 if the temperature and possibly dther factors which change 

 with the seasons, is above a certain minimum at the time of the 

 moult. If such animals were moulting all the year round, they 

 would gradually become darker-coloured in the spring and 

 through the summer, and they would gradually lighten-up 

 again in autumn and winter. As it is, there are animals which 

 moult twice a year at definite seasons, once when the temper- 

 ature is low, and once when it is high. Therefore a drawer of 

 skins of such animals collected throughout the year exhibits a 

 discontinuous variability. Very striking instances can be seen 

 in the seasonal dimorphism of some species of bivoltine butter- 

 flies. Vanessa prorsa and Vanessa levana are strikingly differ- 

 ent. A lot of Vanessa's comprising both spring and autumn- 

 form exhibits discontinuous variability. In nature intermed- 

 iates are not found. The springfoim passes the pupal stage in 



