INTRODUCTION 7 



that confer on them an advantage ? This matter will be 

 discussed at greater length at a later stage in this work. 



(iii) On surveying the general field of variation in all its 

 aspects the first impression one gains is of the very great 

 plasticity of animals. This is, it is true, more clearly seen 

 in some groups than in others, but marked variability 

 is very general. Nevertheless variation is subject to strict 

 limitations. The living organism is not capable of variation 

 in all degrees and directions. Pantin (1932, p. 710), in an 

 interesting essay, refers the limitations of variation to the 

 fact that protoplasmic materials comprise a limited number 

 of standard parts of limited properties. In spite of the seem- 

 ingly infinite plasticity of morphological parts the variation 

 of the living substance is limited by the character of its mole- 

 cular structure. Thus Pantin (I.e. p. 709) cites the fact that 

 only four respiratory pigments have been evolved capable 

 of combining reversibly with oxygen. He suggests that 

 the same limitation affects the capacity for morphological 

 variation. We might explain on these lines the very notable 

 occurrence of parallel evolution and the development of 

 similar variation in allied species. 



The limitations of variability in a particular group of 

 animals (Dinqflagellata) has led Kofoid (1906, pp. 251-2) to 

 stress the analogy between the variation of a group of ' ele- 

 mentary species ' and a group of related organic compounds. 

 ' The seeming reversion in these mutants (?) of Ceratium to 

 old and fundamental subgeneric types, the occasional rever- 

 sibility of mutations elsewhere and the limitations in the range 

 and number of mutant types appearing in nature and under 

 culture suggest that the chemical nature of living substances 

 . . . place certain rather definite restrictions upon the number 

 and amplitude of the departures which mutants make from 

 their sources . . . the relation which exists among the mem- 

 bers of a group of elementary species . . . presents a striking 

 analogy to that which is found to exist among the various 

 radio-active substances or members of a chemical series of 

 related organic substances.' 



In the preceding paragraphs we have considered the origin 

 and nature of variation, and for the purpose of defining our 

 particular problems it is now desirable to discuss a little more 

 fully the way in which variants occur in nature. 



At the offset the exact study of natural variation is rendered 



