102 ^\}:f0$$^E&I$G OF ANIMALS 



five at a birth have been reported. When twins are 

 born they are either of identical sex or one a male and 

 the other a female. In some cases the twins are very 

 much alike in all other characters as well as sex. Such 

 twins were called by Galton identical twins. It is also 

 true that twins are often born which have no greater 

 resemblance to one another than ordinary brothers and 

 sisters. Such twins undoubtedly develop from separate 

 eggs and are known as ordinary or fraternal twins. They 

 do not necessarily resemble one another more closely 

 than brothers and sisters of the same family except 

 that they are of identical age and for this reason might 

 be expected to have a closer resemblance than brothers 

 or sisters of widely different ages. Fraternal twins may 

 be of different sex. Identical twins are believed to come 

 from one egg after fertilization. They are always of the 

 same sex and very much alike in external and internal 

 character and in mental and moral tendencies. 



101. Characters correlated with fertility. It is in the 

 highest degree desirable that the breeder should be able 

 to distinguish those qualities, external and internal, 

 which are in any way correlated either with fertility or 

 sterility. Unfortunately we cannot now speak with as- 

 surance on all the supposed evidences of fertility in 

 animals, but some characters are undoubtedly closely 

 correlated with fertility and we may through them learn 

 to judge of the probable existence or nonexistence of 

 this most desirable trait. Manifestly, characters closely 

 correlated with fertility will finally persist and become 

 dominant. It is equally evident that those characters 

 of the animal body which are correlated with infertility 

 will ultimately disappear. The skillful judge of breed- 

 ing animals recognizes some such correlation in the selec- 



