. VARIATION. 273 



Now, this theory is difficult of explanation in regard to the 

 appearance of double-headed worms, unless we take the possi- 

 bility that the potentiality of a Double Individual in the ovum, 

 for some inexplicable reason, has been delayed until the time of 

 the formation of the head region, thus giving only an approach 

 to the possible double individual which was prophesied of in 

 the original ovum. 



On the other hand, division of the meristematic cell may 

 explain the problem, and this, as yet, is by far the most likely. 

 Still the question remains an open one. 



Bibliography. 



1. Darwin's ' Origin of Species.' 



2. Darwin's ' Earthworms and Vegetable Mould.' 



3. Bateson's ' Materials for the Study of Variation,' 1904. 



4. Thomson's ' Evolution.' 



5. Weismann, ' Heredity.' 



6. Wilson, ' Anatomischer Anzeiger,' vii. 1892, p. 732. 



7. Driesch, «Zt. f. w. Zool.' 1891, vol. iii. p. 160. 



Zool. Uh ser. vol. XVII., July, 1913. 



