68 ANIMAL INDIVIDUALITY [OH. 



though most of their relations may grow up in the 

 ordinary humdrum way " one egg, one adult." 



Aberrations may occur even in man : there can 

 be very little doubt that identical twins 1 (to leave 

 all double monsters out of account) arise from the 

 two cells produced by the first division of a single 

 fertilized ovum, which have accidentally been torn 

 apart instead of staying united. 



A very interesting variation on this is seen in 

 the nine-banded armadillo (Dasypus novem-cinctns) 

 which regularly produces "identical quadruplets" (14). 

 Most mammals give birth to several young at one 

 time, but usually each grows up from a separate and 

 separately fertilized ovum and each is enclosed in its 

 own set of embryonic membranes. The armadillo's 

 brood, however, like the identical twins in man, has 

 only a single chorionic membrane, and the four re- 

 semble each other minutely. Always of the same sex, 

 their measurements are identical ; even the number 

 of plates in their armour is constant to less than 1 

 per cent., though the range of variation from brood 

 to brood may be 5 per cent, and more 2 . 



1 It is well known that there are two kinds of twins: identical 

 twins, always of the same sex and almost indistinguishable from each 

 other, and ordinary twins, which may be of opposite sexes, do not 

 resemble each other more closely than brothers of different ages, and 

 like them arise from the fertilization of two separate ova by two 

 separate spermatozoa. 



2 It is an interesting fact that the four twins fall naturally into 



