68 VARIATION 



Fig. 1 8 shows a case of double head in the turtle. Many 

 similar instances have been described, but this is especially inter- 

 esting because " the two heads seemed to act independently, and 

 it is said there was no concerted action between the feet of the 

 two sides." The same phenomena of double monsters are said 

 to be frequently noted in fish-hatching establishments. Among 

 snakes " some twenty cases are recorded of complete or partial 

 duplicity, nearly always of the head. Several of these were ani- 

 mals of good size, and must have led an independent existence 

 for some considerable time." 1 



Similar cases of doubling are known in birds and even in mam- 

 mals, but among these higher animals the practical difficulties 

 in sustaining existence with extreme abnormality are very great, 

 and they commonly do not long survive. 



Between this division of a single organ lying on the median 

 line and the doubling of so important a part as the head, there 

 seems to be no clear line of demarcation. This doubling may 

 even go further, as in the case of the Siamese twins, until the 

 specimen is regarded as essentially two individuals united by 

 some sort of attachment. 



SECTION IV SYMMETRY IN VARIABLE PARTS 



Without a doubt meristic variation in one organ of the body 

 is likely, but not certain, to be accompanied by abnormality in 

 another. For example, a variation among the digits of the fore 

 foot is likely to be associated with a similar variation behind, still 

 more likely on the opposite side, but not positively with either. 



Again, there is some suggestion of symmetry within the part 

 itself in which the variation occurs. A good example of this is 

 Bateson's No. 495 (Fig. 19). 



This is a left hand, and the four extra fingers seem to repre- 

 sent not the thumb of that hand but the fingers of the opposite 

 (right) hand, thus seeming to aim at a kind of secondary sym- 

 metry within the member. 2 



In the description of this case we are told that this double 

 hand and arm were very muscular, so that it was not possible 



1 Bateson, Materials, etc., p. 561. 2 Ibid. p. 335. 



