VARIATION 427 



what is called orthogenesis, or progressive variation along a 

 definite line. The palaeontologists in particular have very 

 strong convictions as to reality of this orthogenesis, and as to 

 the absence of arrows shot at a venture. (B) Instances are 

 accumulating of the occurrence of mutations or brusque varia- 

 tions, and if these come they often come to stay. The Black 

 Mutant of the Peppered Moth was rare 60 years ago; in 

 many places it has now replaced the originative stock. This 

 lessens the element of the casual in organic evolution. It 

 also lessens the need for over-burdening the role of natural 

 selection in sifting out from amid a crowd of random novel- 

 ties, and as an accumulator of minute increments. (C) But 

 along with this there should be considered the idea, that 

 variations are limited in some measure by what has gone 

 before. At the beginning of each individual life there is the 

 fertilised ovum, a viable unity. If a variation occur it 

 is not like to grip unless it be congruent with the germinal 

 organisation already established ; it must harmonise, just as 

 an addition to a crystal must, but within a wider range. The 

 character of the building that has been erected determines in 

 some measure the nature of an addition to it. The idea of 

 architecture is of course only one aspect; the novelty must 

 be congruent with the previously established reaction system 

 and specific metabolism. Out of the same spring we do not 

 get sweet water and bitter. This is an old but important 

 idea ; we find in Aristotle the suggestion that the possible 

 range of the form of an organ is limited to some extent by 

 its existing differentiation. Thus the element of the fortu- 

 itous shrinks still further. It is interesting to find that 

 monsters sometimes result from infelicitous crossings, but 

 perhaps a greater interest attaches to the fact that monsters 

 are rare in Nature, not only in survival, but in occurrence. 



