116 BLASTOGENIC VARIATIONS. 



certain proportion exhibit a most striking resemblance, 

 which, although not perfect, is much closer than has 

 ever been observed in children born successively. 

 These Weismann speaks of as " identical " twins. He 

 says there is every reason to suppose that such twins are 

 derived from a single ovum and spermatozoon, whilst 

 dissimilar twins are derived from two ova, which must, 

 of course, have been fertilised by two different sperma- 

 tozoa. If this is actually the case, it furnishes a proof 

 that heredity is potentially decided at the time of ferti- 

 lisation. 



Interesting cases of identical twins have been re- 

 corded by Galton in his book on " Inquiries into Hu- 

 man Faculty." With reference to disease, for instance, 

 it was found that both twins were apt to sicken at the 

 same time in 9 out of the 35 cases collected. Either 

 their illnesses were non-contagious, or, if contagious, the 

 twins caught them simultaneously. The mental and 

 moral resemblance between the twins was just as close 

 as the physical. An instance cited by Dr. J. Moreau * 

 well illustrates this. His case was one of twin brothers 

 who had been confined on account of monomania. They 

 were physically so alike as to be easily mistaken for 

 one another, and as regards their moral condition they 

 had exactly the same dominant ideas; they both con- 

 sidered themselves subject to the same imaginary per- 

 secutions ; they both had hallucinations of hearing ; both 

 were melancholy and morose. 



Unfortunately, Galton did not obtain any exact 

 anthropometric data. Weismann has obtained one 

 series of measurements, however, viz., for twin brothers 

 * "Psychologic Morbide," Paris, 1859, p. 172. 



