24 ILLUSTRATIONS. [introd. 



in the tissues or substance of which the parts are made up. All 

 belong to and are recognizable as belonging to the same sort of 

 Narcissus. On the other hand many Narcissi, i\ r . corbularia, for 

 example, are known in two colours, one a dark yellow and the other 

 a sulphur-yellow, though the number of parts and pattern of the 

 flowers are identical. This is, therefore, an example of a Sub- 

 stantive Variation. 



Again, the foot of a Pig may, through Meristic Variation, be 

 divided into five or six toes instead of into four ; or, on the other 

 hand, the number may, by absence of the median division between 

 the digits III and IV, be reduced to three, though the tissues 

 composing the toes may not in structure differ from the normal. 



Again, the tarsus of a Cockroach (Blatta) may, through Me- 

 ristic Variation, be divided into only four joints instead of into 

 five, the normal number, but the joints are still in substance or 

 quality those of a Cockroach. 



I am aware that Meristic and Substantive Variations often 

 occur together, and that there is a point at which it is not possible 

 to separate satisfactorily the changes which have come about by 

 the one process from those which have come about by the other. 

 Instances of this kind occur especially in the case of series of 

 parts such as Teeth or Vertebrae, in which individual members 

 or groups of members of the series are differentiated from the 

 others. For example, we may see that it is through Meristic 

 Variation that the vertebral column of a Dog may be divided into 

 a number of Vertebra? greater or less than the normal ; and 

 though in such cases all the Vertebrae have distinctively canine 

 characters, yet there are nearly always Substantive Variations 

 occurring in correlation with the Meristic Variations, manifesting 

 themselves in a re-arrangement of the points of division between 

 the several groups of Vertebrae, and causing individual Vertebrae 

 to assume characters which are not proper to their ordinal 

 positions. 



Further inquiry into the questions thus raised cannot at this 

 stage be profitably undertaken, though when the evidence has 

 been considered it will perhaps be advisable to recur to them ; all 

 that is now intended is to indicate broadly the general scope of 

 Meristic and Substantive Variation respectively. 



As has already been stated, it is proposed to begin the Study 

 of Variation by an examination of Variations which are Meristic, 

 leaving the consideration of Substantive Variation to be under- 

 taken hereafter. But nevertheless in the consideration of Meristic 

 Variation it will be necessary to refer to phenomena of Substantive 

 Variation in so far as their occurrence or distribution in the body are 

 affected by Meristic phenomena. For in the determination of the 

 magnitude of the integral steps by which Variation proceeds, the 

 existence of Merism plays a conspicuous part, and it is in con- 



;. 



M 



