42 VARIATION 



5. That forward homoeosis in one region is not necessarily 

 attended by forward homoeosis in other regions of the same 

 series. 



6. That in general (especially in man, where this has been 

 most studied) forward homoeosis is attended by a total increase 

 of the series, and backward homoeosis by a decrease. 



7. That an abnormality on one side may or may not be 

 attended by a like abnormality on the other, though the tend- 

 ency is strongly to the preservation of bilateral symmetry. 



8. That when one part resembles another it is the member 

 lying contiguous ; that is, a dorsal vertebra will resemble a cer- 

 vical or a lumbar, not a sacral, and lying between the stamen 

 and the leaf are the petal and sepal and all intermediate grada- 

 tions, either present or obliterated. 



Meristic variation in spinal nerves. Branches from the spinal 

 cord emerge between the vertebrae, so that in general the sys- 

 tem of spinal nerves is determined by the vertebrae. Aside from 

 this, however, the emergence of the branches varies greatly 

 both in number and in conformation, even when the vertebrae 

 are normal. 



Fiirbringer's 1 studies in birds show that the minimum num- 

 ber of spinal nerves forming the brachial plexus (supplying the 

 wings) is three, but in some species it rises as high as six. 

 Moreover, in some instances the number varied from four to 

 five within a single species, and in one (the pigeon) the varia- 

 tion was from four to six. As might be suspected, the two sides 

 are often differently supplied. For example, in one specimen 

 (goose) " the plexus was formed on the right side by nerves xvi, 

 xvn, xvm, and xix, while on the left side it received a strand 

 from the xxth nerve in addition to these." 



Fiirbringer's tables show that in some specimens of the goose 

 the wings were supplied by the nerves xv to xix, while in 

 others they were supplied by the xvnth to the xxth. In the 

 dove the brachial plexus was formed by the xth branches of 

 the spinal nerve in some specimens, by the xnth to the xvth 

 in others, and in one case by the xith to the xivth as an 

 intermediary. 



1 Bateson, Materials, etc., pp. 129-135. 



