This incredible library with all books of past scientists. For example Ernst Haekel a pecfect one for MagicaVoxel!
Alas I have no time for play with it! http://www.biolib.de/
100 Plates by Ernst Haekel
http://caliban.mpipz.mpg.de/haeckel/kunstformen/natur.html Have fun voxel modeling!

From a little part this image http://caliban.mpipz.mpg.de/haeckel/kunstformen/Tafel_085_300.jpg
I don't want imagine the infinite exploration of only the 100 plates! :)