MERISTIC VARIATION 37 



the two extremities at once very different, and forestalls the 

 development of any very pronounced symmetry longitudinally. 



Animals in their locomotion establish different relations at their 

 opposite extremities, thus preventing exact symmetry in this direc- 

 tion, and yet reminders of inherent tendencies toward universal 

 symmetry are constantly encountered. Long worms, for example, 

 though distinctly different at the extremities, are yet composed 

 of rings very much alike throughout most of their length, even 

 permitting locomotion backward with considerable facility. 



Longitudinal division, however, with or without corresponding 

 symmetry, is everywhere found both in plant and animal life, espe- 

 cially in the latter, and linear series of similar parts present as 

 many opportunities for variation as are afforded by radial series 

 either with or without bilateral symmetry. Thus the rings of 

 worms, the vertebrae and the ribs of the body, the joints of the 

 fingers and of insect parts, all these are fertile sources of 

 meristic variation. 



Homoeosis in meristic variation. This is a form of variation in 

 which one part assumes the characters or appearance of another, 

 usually quite distinct. It is a frequent accompaniment of meristic 

 variations. For example, an extra vertebra may be found in the 

 dorsal series, increasing the number by one, all normal. This 

 is meristic variation of the simplest kind, with no homceosis. 



On the other hand, it may be situated at the front of the dor- 

 sal series and partake somewhat of the character of a cervical 

 (forward homceosis), or it may be located at the rear of the dor- 

 sal series and in many respects resemble a lumbar (backward 

 homceosis). 



This posterior dorsal vertebra may bear a rib that is bifurcated 

 at the extremity, one branch effecting a union with the sacrum, 

 the other floating, in which case there is doubt as to the real 

 character of the additional vertebra, whether dorsal or lumbar. 



In much the same way misplaced organs are often found. A 

 leaf may be seen growing from a fruit, an antenna may spring 

 from an injured eye, or foot appendages may develop instead of 

 those proper to the extremity of the antenna. 



All cases of this order, in which one organ through some dis- 

 turbance assumes the character of another organ, are known as 



