A Source Book in Animal Biology

able according action already animals appear arteries become birds blood body brain called cause cells certain changes character common compared complete considerable considered contained continued direction disease earth effect elements entirely equal evident existence experiments external extreme fact fluid force four function give given greater hand head heart Hence idea important increase individual kind known laws least less light living manner material matter means membrane mind motion move movement muscles namely nature necessary nerves never object observed occur organs origin pass perfect phenomena plants possible present probably produced question reason regard region relation remains result seems seen sense separated side similar species structure substance sugar things tion Variation variety various vegetable veins vessels whole