66 



VARIATION 



Conversely, impaired organs standing on the median line may 



be divided so as to form a pair of organs symmetrically placed. 

 It should be noted that in general a single organ standing on 



the median line, as the nose, is symmetrical both with reference 



to itself and to the median line, but 

 that for the most part paired organs, 

 though symmetrical with reference to 

 the median line, are not themselves 

 necessarily symmetrical bodies (ears, 

 arms, hands). In other cases of paired 

 organs, however (eyes, kidneys), the 

 members do have a kind of symmetry 

 of their own. 



Again, nothing is more common, 

 especially among plants, than to find 

 a single organ on the median line 

 appearing as a paired organ in cer- 

 tain individuals or in nearly related 

 species or varieties. 



Bateson gives as examples of the 

 last the posterior petal in Veronica, 

 which in most related species appears 



FIG. 17. Compounding of paired as a pa i r o f petals lying on either side 



organs: the two horns of this . . . , ,, ,. 



roebuck are united into a single of the middle lme - 



beam for a considerable dis- After giving numerous instances of 

 tance, but afterwards they sep - division of median organs in fishes 



arate. After Bateson . . . . . r 



and in insects, he cites authority tor 



saying that " The organs most often divided in man are the 

 sternum, neural arches, uterus, penis, etc., and of these, speci- 

 mens may be seen in any pathological museum. 1 Organs more 

 rarely divided are the tongue, epiglottis, uvula, and central neural 

 canal." 2 These latter are in reality cases of axial duplicity. 3 



1 Bateson, Materials, etc., pp. 450-458. 



2 Teratology is that branch of biology which treats of abnormalities, and it 

 affords many cases of extreme variations. This study has been considered as 

 curious rather than profitable, and yet, as such abnormalities are coming to be 

 regarded as frequently due to a defective germ, it may yet prove that attention to 

 cases of this order may furnish the key to the solution of questions involving the 

 unit of variability. 3 Bateson, Materials, etc., pp. 559-566. 



