2 THE BIOLOGY OF TWINS 



the sex of twins we shall avoid for more reasons than 

 one the terms "homosexual"' and "heterosexual," 

 which refer respectively to twins both of the same sex 

 and twins of opposite sexes, and shall use the less 

 objectionable terms "same-sexed" and "opposite- 

 sexed." 



Where the use of technical terms appears unavoid- 

 able, the reader will find that, as a rule, a term is defined 

 when first used and possibly in more places than one. 

 Frequently, too, when the verbal description is difl&cult 

 of comprehension the illustrations will give the essential 

 information. 



In this book an attempt is made to gather from many 

 sources the facts about mammalian' twins and to unify 

 these varied situations into one point of view. My 

 own interest in this subject has grown out of eight 

 years' study of what is perhaps the most striking case 

 of twin-production known: that exhibited by the nine- 

 banded armadillo of Texas {Dasypus novemcinctus) . 

 The somewhat disproportionate space devoted to the 

 phenomenon of polyembryony in this species needs no 

 apology. The various aspects of its biology have been 

 more extensively studied than those of any other 

 species, and an author may be forgiven for emphasizing 

 the parts of his subject with which he has a first-hand 

 acquaintance. 



' The term "homosexual" is extensively used in the literature 

 dealing with abnormal sex relations and is therefore pre-empted. 



' An extensive chapter on twinning among the various vertebrate 

 classes below the mammals would appear to lend completeness to this 

 volume, but a review of the extensive literature convinces me that such 

 a chapter would be more confusing than helpful' to the general reader. 

 I shall therefore deal with twinning in mammals only. 



