In 1909, hemoglobin crystals from >100 species were used to relate taxonomy to molecular properties.[6] That book was cited by Perutz in the 1938 report[7] of horse hemoglobin crystals that began his long saga to solve the crystal structure. Hemoglobin crystals are pleochroic - dark red in two directions and pale red in the third[6] - because of the orientation of the hemes, and the bright Soret band of the heme porphyrin groups is used in spectroscopic analysis of hemoglobin ligand binding.