DISCONTINUITY AND COLOUR xvii 



2. The Subjective Element in Judging of Discontintiity. 



The amount of discontinuity in a series of living forms 

 cannot be inferred with safety from superficial appear- 

 ances. Notwithstanding the statement of R. H. Lock, 1 

 especial caution is necessary when the differences are 

 those of colour. Discontinuous colour impressions are of 

 course due to different rates of vibration in the different 

 parts of a continuous series. The discontinuity is in our- 

 selves, and not in the object. It is in every way probable 

 that the chemical changes by which a pigment is trans- 

 formed are excessively minute, although the impression 

 produced on our senses is so great. A change in the 

 colours of a butterfly's wing would doubtless appear as an 

 important discontinuity, while a slight modification of 

 venation in the same wing might well appear as an 

 example of continuous evolution. And yet it is well 

 known that a very small change in venation more truly 

 represents specific difference than does the largest change 

 in colour. 



3. De Vries's Evidence in Favour of Mutation. 



Although the term Mutation might just as well have 

 been applied to evolution of any kind, even the slowest 

 and most imperceptible, it is employed to designate a 

 theory of modification by large and sudden steps. 2 Unin- 

 structed statements— commonly encountered just now in 

 the press — inform the world that Natural Selection is 

 entirely dispensed with by modern writers on Mutation 



1 ' Definite alterations in the colour of offspring as compared with their 

 parents seem almost necessarily to be of this nature ' : viz. discontinuous. 

 I.e., p. 124. 



2 ' Species arise by mutation, by a sudden step in which either a single 

 character or a whole set of characters together become changed.' 

 Variation, Heredity and Evolution, R. H. Lock, London, 1906, p. 144. 



POULTON D 



