CHROMOSOME THEORY OF HEREDITY 247 



third neutralised by the products of others. Each environment 

 will strike a different combination of genes. This conception, 

 startling as it appears at first sight, is paralleled in other branches 

 of biology. Thus, the secretin produced by the small intestine 

 circulates through the whole body ; but all the tissues remain 

 " dumb " to it with the exception of the pancreas. 



From this standpoint, the unity of the organism as a whole 

 is not only explicable, but necessary. Each cell represents a 

 particular state of equilibrium, and the organism as a whole an 

 equilibrium of all the cells with each other and with environ- 

 ment. Remove one type of cell and cultivate it alone, as in 

 tissue-culture experiment ; its type will be altered. Remove 

 part of the whole ; the working of the rest will be changed. 



The development of an organism is a series of states of 

 equilibrium, usually of increasing complexity, none fully 

 balanced, but each resolving itself automatically into the next. 

 Finally, the adult state is reached, in which relative stability 

 is assured, either by the narrow limits of the environment, or 

 by elaborate regulatory mechanisms, of which the endocrine 

 glands are the best example. 



A great change has come over our ideas on the subject 

 since Weismann's day. Weismann's merit {The Evolution 

 Theory, and other works) lay in destroying the old uncritical 

 attitude towards the inheritance of " acquired characters " 

 or modifications ; in formulating the idea of determinants ; 

 and in locating them in the chromosomes, thus rendering the 

 function of mitosis and the reducing division intelligible. 



His demerits were his over-emphasis of the distinction 

 (which is none the less usually a real one) between soma and 

 germ-plasm ; and his total failure to construct a physiological 

 theory of development — a failure which led him into an unreal 

 symbolism. 



He never tried to think except in terms of determinants. 

 Accordingly, when a salamander regenerated a limb, or a 

 bisected planarian became two perfect animals, he was forced 

 to postulate a whole battery of reserve-determinants, ready 

 to come into play if the accident of injury should befall the 

 organism. 



It was soon seen that this, even as the merest formal 

 explanation, was untenable. On the other hand, the idea of a 

 complete self-reproducing gene-complex in every cell of the 

 body is fully adequate, and indeed is the only conception save 

 that of a mystical vital force or of an unanalysed constitution 

 or tendency, which can account for the facts of regulation 

 and regeneration. 



After the Planarian has been bisected, the equilibrium of 

 either half is different from that of the original whole. Changes 



