248 SCIENCE PROGRESS 



may therefore occur, and, since the whole battery of genes is 

 present, the missing organs may be restored under new condi- 

 tions. If we regard the normal form as a condition of balance, 

 it is no more remarkable that what is restored is just what has 

 been taken away, than it is for the form-equilibrium of a 

 damaged crystal to be restored when placed in an appropriate 

 solution. 



Innumerable complications exist. Some cell-differentia- 

 tions appear to be irreversible ; whether regeneration shall 

 occur or not is much influenced by age, temperature, size, and 

 other conditions affecting metabolism ; de-differentiation to a 

 primitive condition may be necessary before regeneration can 

 occur ; and so forth. But the main concept holds. Some 

 examples may clarify it. As is well known, the lens of the 

 eye in Vertebrates is formed from the ectoderm of the head, 

 at the point where the optic cup approaches the skin. If the 

 developing optic cup of a salamander larva be removed and 

 placed under the skin of the abdomen, no lens forms in the 

 accustomed place in the head, but one is produced (see Morgan, 

 Regeneration, p. 204) over the transplanted optic cup from the 

 abdominal ectoderm. Professor MacBride finds this a difficulty 

 for the determinant theory, since it presupposes that " lens- 

 determinants " must exist in every cell of the ectoderm. So, 

 presumably, they do ; and in every cell of the body too. But 

 only in the ectoderm have the conditions been such as to bring 

 the cells into the state in which lens-substance is ready to be 

 produced ; and it will only be produced under the action of a 

 specific substance secreted by the optic cup. That seems the 

 natural way of interpreting the phenomenon. The " lens- 

 determinants," whether one or many, and in any event acting 

 in co-operation with a vast number of other genes, are dumb 

 notes, except in ectoderm-cells struck by the optic cup's 

 specific substance. Elsewhere they do not act ; or, more 

 probably, act in different ways to produce totally other results. 



For we must guard ourselves rigorously from the error, 

 elementary, yet none the less insidious, of supposing that each 

 gene " represents " some single adult character in the same way 

 that a word may represent a particular object, or the indenta- 

 tions on a gramophone record represent each a special sound. 

 Every character, such as eye-colour, stature, longevity, needs 

 the co-operation of several genes, and each gene probably 

 affects a multiplicity of characters. This has emerged clearly 

 from all recent Mendelian work, and nowhere more clearly 

 than in Drosophila. To give but two examples : the red eye- 

 colour of Drosophila is affected by a large number of separate 

 genes, each producing a characteristic effect ; while the factor 

 for yellowness in mice, if present in single dose, not only extends 



