SECT. XI.] 



INTRODUCTION. 



Gl 



A form with four segments occurs as the offspring of a form 

 with three segments. . v Such a Variation, then, is discontinuous 



Fig. 7. Diagram of the flower of a Tulip having all the parts in -4. 



because a new character, that of division into four, has appeared in 

 the offspring though it was not present in the parent. This new 

 character is a definite one, not less definite indeed than that of 

 division into three.^ It has come into the strain at one step of 

 Descent. Instances in which there is actual evidence of such descent 

 are rare, but there can be no question that these changes do 

 commonly occur in a single generation, and, indeed, in many 

 plants, as for example Lysimachia (especially L. nemorum), flowers 

 having all the parts in -4 or in -6 may be frequently seen on 

 plants which bear likewise normal flowers with the parts in -5,. 



Now such a variation as this of the Tulip illustrates a pheno- 

 menon which in the Study of Variation will often be met. 



<We have said that the variation is discontinuous, meaning 

 thereby that the change is a large and decided one, but it is more 

 than this ; it is not only large, it is complete?} 



The resulting form possesses the character of division into four 

 no less completely and perfectly than its parent possessed the 

 character of division into three. The change from three to four is 

 thus perfected : from the form with perfect division into three is 

 sprung a form with perfect division into four. This is a case of a 

 total or perfect Variation. 



This conception of the totality or perfection of Variation is one 

 which in the course of the study will assume great importance, 

 and it may be best considered in the simple case of numerical and 

 Meristic Variation before approaching the more complex question 

 of the nature of totality or perfection in Substantive Variation. 



The fact that a variation is perfect at once leads to the ques- 



