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 (¢.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,’ 
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. 
