Haeckel stressed the physical similarity
of humans and animals and considered human
thought as just a physiological process. His
comparative embryology lowered humans
from a special creation to simply members
of the animal kingdom. Haeckel’s Wonders
of Life (1904) was a supplement to his bestselling
Riddle. It declared that a newborn
human is deaf and without consciousness,
from which Haeckel reasoned that it had no
soul or spirit at birth. Haeckel advocated
the destruction of abnormal newborn infants
and invalids. He referred to this as ‘an act of
kindness’: