GERM PLASM 13 



clone or the sexual reproduction of an inbred race, it is always 

 found that the likeness is imperfect ; further, the lack of likeness is 

 not inherited, but is evidently due to the effect of different external 

 conditions on the organism. This principle was first clearly 

 recognised by Louis de Vilmorin. It was expressed by Weismann 

 in the notion that the likeness was due to the constant properties 

 of the germ-plasm, which was handed down unchanged from 

 generation to generation. Finally, it was developed by Johannsen 

 (1911, et at.) into the notion that the properties of an individual 

 were the product of the reaction of its genotype, or type of germ- 

 plasm, and the environment. On this notion genetical theory is 

 built. 



Environment requires a word of explanation. A fern may 

 abnormally produce a prothallium vegetatively, and although the 

 two must be supposed to have the same genotype their growth is 

 entirely different. The reason is that although the two are separate 

 individuals their development is conditioned by the circumstances 

 of their origin, and the difference between them is simply a form of 

 differentiation, the method of which is, like all other properties, a 

 product of the genotype-environment reaction. So also with the 

 various kinds of life cycle in the Protista : these are in different 

 degrees subject to the variation in external conditions, but the 

 different degrees of this subordination are to be considered a property 

 of the genotype. Thus the visible properties of the organism (its 

 phenotype) are the product of the reaction of its genotype with its 

 environment. Further — and this is most important in the present 

 study of cytology — differences in the properties of individual cells 

 and nuclei of organisms may be referred to one of four determining 

 conditions : (i) differences of genotype ; (ii) differences in relation 

 to development or life-history (i.e., in the differentiation in space or 

 time) ; (iii) differences in environment. A fourth condition will 

 later be seen to be important in the special circumstances of germ- 

 cell formation in hybrids — the occurrence of differences between 

 corresponding chromosomes. These conditions must constantly be 

 borne in mind in attempting to analyse the behaviour of 

 chromosomes. 



When therefore a particular character in an organism is said to 



