2 5 2 THE POPULAR SCIENCE MONTHLY 



up the individual life history. 4 It is not impossible that other segrega- 

 tions than those of the chromosomes form part of the germ-cell be- 

 havior, but of this we know nothing as yet. In any event, the principle 

 of segregation of actual visible elements of the germ cells has a firm 

 anatomical basis. 



It must not be forgotten that the germ is the entire organism and 

 that it passes through development as the same individual; con- 

 tinuity of individuality is preserved throughout development. There- 

 fore, if we discard determinant hypotheses and take our stand on 

 a strictly physiological theory of development, it follows of necessity 

 that the transmitted factors of heredity included in the organization 

 of the germ cells must be factors in the development of the entire 

 organism. The so-called Mendelian factors must therefore be of this 

 character, as I have argued elsewhere. That is to say, the segregated 

 factors must be general constitutional conditions effective as factors 

 in the development of every part of the organism. It can readily be 

 seen that specific intensity of metabolism, or of reactivity, and varia- 

 tion in constitutional size of cells may be such conditions. Others no 

 doubt exist, of which sex may be one. The essential thing to recognize 

 is that the heritable and segregable factors, being conditions of the 

 germ cells at the start, can never be anything less than factors of the 

 entire organism at all stages. 



Our conclusion is that the theory of individual development must 

 more and more come to be regarded as a branch of physiology proper. 

 The theory of representative particles must be relegated to the class of 

 formal hypotheses whose usefulness is largely outlived. While it may 

 still play a part in speculations on heredity, I believe that it will come 

 to be generally recognized by those who use it as a mere matter of 

 convenience of terminology, and not as an explanation of the phenomena 

 described in its terms, in the sense of being a verifiable part of the 

 sequence of processes in development. 



* This general argument would stand even if the chromosomes be regarded 

 merely as indices of organization. They at least give us a clue as to what 

 " the organism " is doing at the time in question. This is indeed all we can 

 say of any characters at any period if we consider the matter in a strictly 

 logical sense. 



