Manchester Memoirs, Vol. I. (1906), No. 11. 3 



this disease will flourish. That which is necessary, there- 

 fore, to make progress both surer and swifter is a greater 

 aptitude to formulate a clear idea of the meaning of the 

 terms which are employed — a habit of mind which is not 

 likely to be common so long as the consensus of biological 

 opinion regards with less favour the attempt to discover the 

 essence of a newly suggested hypothesis, than the attempt 

 to describe the course of the vas deferens in a newly 

 discovered worm. 



In the study of heredity in particular the most 

 extraordinary confusion has resulted from the fact that 

 not only has the same term been used to mean different 

 things by different writers, but very often has had many 

 significations in the writings of a single author. This 

 state of affairs is due, in my opinion, to the absence 

 of any patient and laborious attempt to thresh out the 

 meanings of the terms continually on the lips of those 

 who take part in the discussion of this subject ; and 

 this absence is due in its turn to the callousness, if not 

 disfavour, with which such an attempt is likely to be 

 regarded. Nevertheless I propose to make it. 



Space forbids me to discuss the question of the 

 advisableness of using the term " law " at all as summaris- 

 ing vital phenomena, more than to say that the fact that 

 I use it 166 times in this paper demands some apology. 



I use it because, besides possessing the advantage of 

 brevity, it is of all terms in biology the vaguest ; signify- 

 ing as occasion demands either a theory, or a resumd, or 

 a hypothesis, or a formula, or a generalisation — to name 

 a few of the more or less legitimate senses in which it is 

 used : and because it shelters, under its wide roof. Laws 

 whose authors aim at explanation, and those whose 

 authors are satisfied with description. And I spell it with 



