70 PROBLEMS OF GENETICS 



chanical processes of division remain a distinguishable group of 

 phenomena, and variations in the substances to be distributed in 

 division may be independent of variations in the system by which 

 the distribution is effected. 



Modern genetic analysis supplies many remarkable examples 

 of this distinction. When formerly we compared the leaves of a 

 normal palmatifid Chinese Primula with the pinnatifid leaves 5 of 

 its fern-leaved variety we were quite unable to say whether the 

 difference between the two types of leaf was due to a difference 

 in the material cut up in the process of division or to a difference 

 in that process itself. Knowledge that the distinction is deter- 

 mined by a single segregable factor tends to prove that the 

 critical difference is one of substance. So also in the Silky fowl 

 we know that the condition of its feathers is due to the absence 

 of some one factor present in the normal form. We may con- 

 ceive such differences as due to change of form in the successive 

 "waves" of division, but we cannot yet imagine segregation 

 otherwise than as acting by the removal or retention of a material 

 element. Future observation by some novel method may suggest 

 some other possibility, but such cases bring before us very clearly 

 the difficulties by which the problem is beset. 



In another region of observation phenomena occur which as 

 it seems to me put it beyond question that the meristic forces are 

 essentially independent of the materials upon which they act, 

 save, in the remoter sense, in so far as these materials are the 

 sources of energy. The physiology of those regenerations and 

 repetitions which follow upon mutilation supplies a group of 

 facts which both stimulate and limit speculation. No satis- 

 factory interpretations of these extraordinary occurrences has 

 ever been found, but we already know enough to feel sure that 

 in them we are witnessing indications which should lead to the 

 discovery of the true mechanics of repetition and pattern. 

 The consequences of mutilation in causing new growth or perhaps 

 more strictly in enabling new growth to take place, are such that 

 they cannot be interpreted as responses to chemical stimuli in 



5 It is a question whether the dominance of the palmatifid leaf over the pin- 

 natifid is not really an example of the dominance of a lower number of segmentations 



