280 PLANT PHYSIOLOGY 



offspring, and second, the continuity of influences to which 

 organisms are exposed. These means are continually em- 

 phasized and for the most part ignored, respectively. The 

 former are for morphology to describe, the latter are within 

 the province of physiology to discuss. Granting the con- 

 tinuity of substance, which we have already considered 

 (pp. 255-63), what are the influences which are continu- 

 ous? 



The environment of an organism is made up of many 

 factors, some great, some small, as judged by their visible 

 effects, some continuous, some constant, some variable, 

 some occasional, some periodic. Among contiuuous factors 

 the following may be mentioned. First, the atmosphere, the 

 composition and pressure of which are unchanging. Second, 

 water, which has the same composition, the same solvent 

 power, and the same carrying power always. Third, grav- 

 ity, a force of uniform strength. Fourth, the earth as a 

 whole, the character of which changes only with inconceiv- 

 able slowness. If these factors are not absolutely continu- 

 ous, taking infinite time into account, they are continuous 

 and unchanging so far as millions of generations of organ- 

 isms are concerned. They are exactly the same for the 

 offspring as for its parents. 



On the other hand, light and heat are forces which act 

 periodically with great regularity, but which are constant 

 only within rather wide limits. Of the influences which are 

 occasional, other and motile living organisms are perhaps 

 the most striking. 



The continuous and the periodic influences are the ones 

 which have received least attention from those interested in 

 the problem of heredity. These influences must conserve if 

 the variable and occasional influences introduce differences. 

 But as a basis for all conceptions of heredity and variation, 

 we must concede the irritability of living protoplasm. 



The egg, which is a part of the living substance of the 

 mother, is penetrated by the spermatozoid, a part of the 

 living substance of the father, and these two fuse into one 

 mass of living substance. Each particle of living substance, 

 while still within the body of the parent, was influenced by 



