254 CAUSES OF VARIATION 



quality is often fatal, as in the case of moths. In general, how- 

 ever, matters have long since become adjusted to these reactions 

 as to other necessities governing the behavior of living matter. 



2. All experiments indicate a high degree of variability among 

 individuals, not only as regards the degree of response to heliot- 

 ropism, but also as regards the effect of all other outside influ- 

 ences, even that of poisons. 



Among examples furnished by the extended investigations 

 into this subject, we have space to note but two. 



The amoeba, which represents about the simplest form of 

 animal life, is an excellent medium for illustrating sensitiveness 

 to light. Fig. 28 exhibits the movements of one of these bits of 

 living matter under the influence of light, whose direction, as 

 shown by the arrows, was occasionally changed, the figures in- 

 dicating the hour, all of which is strongly suggestive of the 

 process of driving sheep. 



The other example to be noted is the effect of light upon the 

 position of the chlorophyll granules in the leaf cells, under dif- 

 ferent degrees of illumination, whether on the exterior walls, the 

 partition, or the interior walls. 



Conditions that determine heliotropism. Some organisms are 

 positively heliotropic in one intensity and negatively so in 

 another ; some are positive at one temperature and negative at 

 another ; some are positive in a certain concentration (of sea 

 water) and negative in another, and the general principle may 

 be stated that decreasing the concentration has the same effect 

 as increasing the temperature. 1 It thus appears that the same 

 organism can often be made positively or negatively heliotropic 

 at will by altering the surrounding conditions of life. 



SECTION VII INFLUENCE OF TEMPERATURE UPON 

 LIVING MATTER 



The relation of heat to living matter is mainly, but not exclu- 

 sively, quantitative ; that is to say, the effect of heat is princi- 

 pally upon the rate of growth and activity. In general, each 

 species has its maximum, above which protoplasm becomes 



1 Loeb, Studies in General Physiology, pp. 265-294. 



