202 The Origin of Species by Mutation. 



neither to the right nor to the left so as to form a zig-zag 

 line.-^ 



The question we have to answer now is how can such 

 a definite and apparently predetermined series of changes 

 be explained by natural principles and especially by the 

 principles of descent enunciated by Darwin ; in other 

 words : how must we picture mutability and natural se- 

 lection to ourselves in order to gain a satisfactory expla- 

 nation of these series. Two ways of explaining them 

 are possible. 



1. iMutability may take place in almost all directions; 

 and it is natural selection which operates in one 

 direction during long geological periods. 



2. Mutability takes place only in one direction and 

 itself determines the direction of change. 



The former obviously represents the view of Dar- 

 win ; the latter that of Scott. 



In the first place it must be remembered that when we 

 are dealing with paleontological facts it is hardly possible 

 to decide between mutability and selection, and, as Scott 

 has remarked, no ''explanation" can ever be much more 

 than a guess. 



There is every reason for supposing that in the gen- 

 ealogy of every organism numlDers of species may have 

 arisen but have never multiplied sufficiently to insure 

 their preservation in the rocks, and have disappeared 

 without leaving behind them either posterity or record. 

 Paleontology can obviously not help us to decide as to 

 the admissibility of such a theory. Let us therefore com- 



^ Weldon regards this objection to the theory of selection the 

 most serious of all. See his Presidential Address : On the Three 

 Principal Objections which Arc Urged Against the Theory of Nat- 

 ural Selection, 8th Sept., 1898. Brit." Ass. Adv. Science, Bristol, 1899, 

 p. 887. 



