496 WHITE— STUDIES OF INHERITANCE IN PISUM. 



In order to secure the greatest uniformity practicable in en- 

 vironmental conditions, all cultures which are studied from a com- 

 parative standpoint are planted in the same batch of soil, at the 

 same time, and given the same cultural care. A few characters of 

 Pisum such as flower color, presence of parchment and presence of 

 tendrils are very little affected by environmental fluctuations. The 

 majority of Pisum characters, however, react to environmental 

 fluctuations so as to give rise to error in any intense study, unless 

 the fluctuations are known well enough to be taken into account. 

 By growing a large series of cultures, both hybrid and pure line, 

 under approximately the same set of conditions by the method men- 

 tioned above, and securing as near as practicable the same condi- 

 tions for several years, one may become so familiar with the factors 

 composing such an environment and the reactions of the plants to 

 such an external set of conditions, that the environment itself may 

 be used as a standard by which the modifications of the same plants 

 grown under other environments may be described. Such an en- 

 vironment may be called a standard environment, as it is the cri- 

 terion by which the effect of all other conditions on characters is 

 decided. Whether such a standard can be made precise enough to 

 be of much value in genetic work remains to be seen. If one de- 

 scribes character changes by revolving round a circle, one gets 

 nowhere, and without a standard starting place, one simply re- 

 volves. The older biologists used the term normal to designate in 

 a vague way what I mean by standard. Normal environment, how- 

 ever, may mean almost any common environment in experimental 

 work. Thus there is no gain in preciseness through its use. 



PART II. 

 The Genetics of Pisum. 



Genetic studies on the genus Pisum may be divided into two 

 groups — those made before and those made since the rediscovery 

 of Mendel's law in 1900. The pre-Mendelian studies resulted in a 

 great deal of practical good, but were of slight scientific value, 

 since no laws of heredity were discovered. The post-Mendelian 

 work is as yet too young to have given great practical results. Laws 



