WHAT IS A "TENDENCY"? 



IN the interesting discussion which took place recently 

 in the columns of Nature on the possibility of the 

 " Inheritance of Acquired Characters," as in all recent writ- 

 ings on the subject of heredity, much use was made of the 

 terms "tendency" and "predisposition". I have searched 

 Darwinian and post-Darwinian literature in the hope of 

 finding an exact definition of these terms, but at present 

 without success. If used vaguely and without any definite 

 meaning, it will be admitted that they have no more value 

 than the "essences" and "humours" of mediaeval natural- 

 ists ; and it seems to me that a clear idea of the meaning- of 

 these terms must be the basis of any fruitful discussion of 

 the phenomena of heredity. 



What is meant by the phrase that an organised being 

 has a "tendency to variation"? Clearly this is meant to 

 express some property or function of the being in question. 

 Now, with regard to the origin of functions, let me quote 

 from Professor Huxley, not because he favours my views 

 on this question, but because he is the clearest thinker and 

 writer of the day on biological subjects : " All functions, 

 intellectual, moral, and others, are the expression or the 

 result, in the long run, of structures, and of the molecular 

 forces which they exert ". 1 - I do not think that the sound- 

 ness of this statement will be generally disputed. The 

 tendency to variation then, in an organism, is a function 

 resulting from molecular forces exerted by the structures of 

 which that organism is composed. The only alternative 

 hypothesis appears to me to be that this tendency is the 

 result of an internal vital force independent of matter ; and 

 this hypothesis can hardly be accepted by those who regard 

 Natural Selection as the sole or the main law which has 

 favoured the development of the different forms of organic 



life. 



1 Darwiniana, p. 472. 



