80 Hereditarji Lc/f/iandedness 



Twill birth is presumably simply a lesser degree (if multiple-birth, 

 characteristic of lower mammals. In general, the higher the degree of 

 development (specialization) the fewer the individuals at a birth. In 

 horses twins are rare ; in certain sheep they are the rule. Fewer 

 individuals at a single birth implies evolutionary advance. From this 

 viewpoints tiie suggested recession of human twin birth to the more 

 normal birth becomes intelligible in terms of the dominance of a derived 

 or advanced, over a racially older, condition. 



The pedigrees given by both Oliver and Davenport show trans- 

 mission through males only. In both of my own histories, the trans- 

 mission is more frequently through the female line. 



On the basis of the experimental evidence from invertebrates, 

 Amphioxus, and amphibia identical (monochorial ; duplicate) twins are 

 commonly explained in terms of a relatively independent development of 

 accidentally separated blastomeres, at the two-cell stage of development. 

 The stimulus or causative agent to separation (or " budding " of em- 

 bryonic disc as in armadillo — Patterson; or "fission," — Assheton) may 

 conceivably be either mechanical or chemical (nutritive — Patterson). If 

 the former, an inheritance of the tendency would hardly be expected. 

 The latter circumstance seems the more probable. The chemical factor 

 for disjunction might inhere either in the male or the female gamete. 

 On this basis heredity of duplicate twins becomes intelligible. 



But occasionally bi-sexual (heterosexual or ordinary) pairs of twins 

 occur in such pedigrees {vide Oliver, Davenport, and Figs. 79 and 80). 

 Ordinary twins are commonly supposed to be due to the practically 

 simultaneous ovulation of both ovaries, or a double ovulation on the 

 part of the functioning ovary. Such tendency may well be hereditary, 

 but it could obviously show itself only in females. It is inconceivable 

 how the spermatozoon could exert influence on the ovary such as to 

 stimulate to double ovulation. Inheritance on the above explanation 

 would have to follow an alternation of generations where males inter- 

 vened ; i.e. only females of such a pedigree could determine twins of the 

 ordinary type. In view of the several pedigrees here involved, it nnist 

 remain doubtful whether such is the case, though pedigrees 79 and 80' 

 are suggestive of this condition. Both ordinary and identical twins 

 appear in the same pedigree, and both conditions seem hereditary 

 according to similar principles, the tendency for both being apparently 

 transmitted either by male or female. It seems in the light of known 



' Twin pairs of the same sex are known to be "identical" only in the instances 

 so specified; regarding the remainder the information was uncertain. 



