1 32 THEORIES OF HEREDITY 



similar effects in the corresponding somatic cells of the 

 offspring. The acquired changes in Q on the right side 

 and V on the left, indicated by their dark colour, would 

 be confined to the organism in which they arose, and 

 would not affect its offspring, at any rate in a corre- 

 sponding manner. 



If the transmission of acquired characters were proved 

 to be an undoubted fact, Weismann's hypothesis of 

 heredity would inevitably collapse. It cannot, however, 

 be maintained that such proof is forthcoming. 



The question largely turns upon an exact knowledge 

 of the proportion borne by blastogenic or inherent to 

 somatogenic or acquired characters. We know how 

 dominant a share of our physical and mental qualities 

 is hereditary — so dominant indeed that it would follow, 

 if Weismann's hypothesis be correct, that blastogenic 

 characters are far more important than somatogenic. 



There is some evidence that this is the case, and 

 I will here bring forward one line of proof, which also 

 supports the conclusion that the whole organism is pre- 

 determined in the ovum. 



If this last conclusion be valid, it follows that the 

 differences which characterize individuals are predeter- 

 mined in the ova from which they arise, and that ova 

 are not in their essential nature alike any more than 

 individuals. We do, however, occasionally meet with 

 individuals so much alike that we (incorrectly) speak of 

 them as 'identical'. The resemblance between certain 

 twins is far closer than that between other members of 

 the same family. If, therefore, we can prove that such 

 ' identical ' individuals are derived from ' identical ' ova, 

 the above-mentioned arguments and conclusions will 

 receive strong support. 



' Identical ' twins are invariably of the same sex. 

 When twins are of different sex, the degree of resem- 

 blance is no greater than that between brothers and 

 sisters generally. This is also true of many twins of 

 the same sex, and Galton has brought forward evidence 

 to show that they may even differ more widely than is 

 usual with brothers or sisters. 



