162 SECRETS OF ANIMAL LIFE 



what doubtless occurs in man; duplicate or iden- 

 tical twins develop from one egg; dissimilar or 

 fraternal twins develop from two separate eggs. 

 Outdoing the poly-embryony of the Nine-banded 

 and the Hybrid armadillos is that of some Hymeno- 

 pterous insects (e.g. Litomastix) which lay their 

 eggs in caterpillars and the like, for from each egg 

 there develops a legion of embryos. One would like 

 also to speak of the twinning of a common earth- 

 worm, which seems to occur most frequently in 

 warm weather, a fact to be taken in conjunction 

 with the experimental result that the eggs of some 

 sea-urchins often twin in large numbers when the 

 water is artificially warmed. Fascinating in its 

 way, but taking us into rather deep waters, is the 

 case of the "free-martin," a sterile and abnormal 

 female calf showing some masculine features which 

 seem to be due to hormones borrowed from its male 

 co-twin, the two of them arising, undoubtedly, from 

 two egg-cells. 



Another point of great interest in the biology of 

 twins was raised long ago (1876) by Mr. Galton, 1 

 who sought to find in the history of twins a criterion 

 of the relative powers of " nature " and "nurture." 

 Will duplicate twins become unlike if subjected 

 to diverse nurture? Will dissimilar twins become 

 more like one another if subjected to the same 

 nurture? Mr. Galton investigated about eighty 

 cases of close similarity between twins, affecting 

 color of hair and eyes, height and weight, strength 

 1 Journal of Anthropological Institute, 1876. 



