196 ZOOLOGY 



the average. Similarly, among plants, smallness may 

 be due to growth under unfavorable conditions, such as 

 lack of moisture ; or may be (as in the case of the dwarf 

 sweet pea) an inherited character. Among sunflowers, 

 the seeds of large kinds produce dwarfs when grown in 

 shade, but no amount of sunshine will make the small 

 kinds grow tall. Among ourselves, we are continually 

 puzzled to know whether the qualities of individuals are 

 primarily inherited, or are principally due to favorable 

 or unfavorable surroundings. No one, trying to judge 

 himself, can be quite sure how much to attribute to 

 each of the two factors. Yet the breeder of animals or 

 plants, especially if he can keep many successive genera- 

 tions under observation and experiment at will with 

 environmental factors, may determine the relations 

 between cause and effect with a high degree of accuracy. 

 The experience so gained enables him to form reasonably 

 accurate judgments in many other cases on mere in- 

 spection, or with a limited history to guide him. 

 Constancy 3. Since the selection of large or small (or otherwise 

 ners differing) Paramecia among the members of a pure line 



did not produce any change in the characters of the 

 race, it was held that the hereditary qualities remained 

 constant during the period of the experiment. Experi- 

 ments of this sort were continued long enough, not only 

 with Paramecia but with other organisms, to lead to 

 the conclusion that actual changes in the germ plasm 

 (original variations) were extremely rare, to say the 

 least. This appeared to be equally true of animals and 

 plants ; thus the Vilmorin wheats remained the same 

 after many years of selection. There remained, how- 

 ever, this difficulty - - that since selection could be 

 based only on tangible or visible characters, it was 

 difficult or impossible to choose the deviations due to 



