202 THK PHYSIOLOGY OF REPRODUCTION 



tions to produce new individuals in which the combinations are 

 interchanged, A being associated with Y, and P> with X. 1 it has 

 been claimed also, that, in spite of the new combinations, each of 

 the original separate unit characters can be preserved in a state 

 of complete purity, and without in any way affecting, or being 

 affected by, the characters upon which they have been superimposed. 

 By resorting to such methods, it has been thought possible to build 

 up, little by little, entirely fresh types of organisms, possessing new 

 combinations of pure characters, which previously existed only in 

 different individuals. 



It remains to be considered how far this conception of an 

 organism as an individual capable of description in terms of unit 

 characters (each of which can be transmitted pure) is in harmony 

 with modern physiological theory, or justified by experimental 

 investigation. 



In the first place, it may be pointed out that the entire trend of 

 physiological research in recent years has been to show that the 

 correlation that exists even between remote parts of the body is 

 often extraordinarily close, and that in all probability there is not 

 an organ or structure that is not dependent in its growth and 

 activity upon chemical substances, elaborated by other and some- 

 times distant parts of the body, and carried thence in the circulating 

 blood. Thus a change in the whole metabolism, producing palpable 

 modification in whole groups of characters, may be induced experi- 

 mentally in the individual, by interfering with or removing one 

 particular organ. This is well shown in the various kinds of 

 correlation existing between the organs of internal secretion. Again, 

 a change in the environment may directly affect the metabolism, 

 ami su influence all the characters of the body. To the physiologist, 

 therefore, a so-called unit character cannot readily be regarded 

 as something represented by a substance located originally in a 

 chromosome or rhromomere. Such a view, as Yerworn 2 remarks, 

 is " toiL morphologically conceived." It is more in keeping with 

 the physiological view of life to regard the characters of the 

 individual as manifestations of a particular kind of metabolism, 

 which is itself partly the outcome of environmental influences, and 

 partly the developmental result of the sort of metabolism that 

 existed in the germ-cells from which the organism was derived. 

 According to this view, it is clear that the presence of any one 

 characteristic may exert an influence upon many, if not upon all, 

 the other characteristics, and that, even in heredity, one cannot hope 



1 The first filial generation is spoken of as the F, generation, the second filial 

 generation as the F. A and so on. 

 '-' ViTworn, /o<: n't. 



