22 VARIATION 



The student must not therefore assume the possibility of inter- 

 mediate gradations and insensible differences when dealing with 

 biological phenomena. Many of these differences are essentially 

 and necessarily discontinuous. It remains to discover which 

 these are and to discover the bearing of discontinuity upon the 

 results to be accomplished by selection. 



If all variations were continuous we might hope to be able 

 theoretically to accomplish any desired result and secure any 

 desired shade of difference by selection ; but if not, then there 

 will remain notable gaps that cannot be filled. The natural corol- 

 lary of all this is that we can accomplish by selection almost any 

 desired shade of result with those variations which are by nature 

 continuous, but that with those variations which are by nature 

 discontinuous, our efforts in this respect will be limited. 



Distinctions arising from the nature of the characters involved. 

 Having determined whether the deviation is quantitative or quali- 

 tative, continuous or discontinuous, we next inquire into the real 

 nature of the variation as it affects the organism. Manifestly 

 this depends upon the character or characters involved. 



Those concerned with form will, in their deviations, give rise to 

 morphological differences. On the other hand, deviation in char- 

 acters distinctly functional will give rise to differences in organic 

 activity without regard to form. 



Accordingly four distinctly different kinds of variation are 

 recognized : 



1 . Morphological, relating to differences in form or size. By 

 nature they are always quantitative, but may be either continuous 

 or discontinuous. 



2. Substantive, relating to differences in quality of the struc- 

 ture as distinct from mere form or size. By definition they are 

 always qualitative and generally, if not always, continuous. 



3. Meristic, relating to deviations in pattern, especially as to 

 repeated parts, as in extra fingers and toes, doubling of petals, 

 stooling of grain, etc. Variations of this kind are either quanti- 

 tative or qualitative, generally the former, but are of necessity 

 discontinuous. 



4. Functional, relating to deviations in the normal activity of 

 the various organs and parts of the body or the plant, such as 



