They occur in vertebrate animals throughout the globe, though varying in abundance in different districts and at different times.
Non-committal as regards evolution, he vastly broadened the field of vertebrate palaeontology by his descriptions of the extinct fauna of England, of South America (including especially the great edentates revealed by the voyage of the " Beagle "), of Australia (the ancient and modern marsupials) and of New Zealand (the great struthious birds).
The minute gradations observed by Hyatt, Waagen and all invertebrate palaeontologists, in the hard parts (shells) of molluscs, &c., are analogous to the equally minute gradations observed by vertebrate palaeontologists in the hard parts of reptiles and mammals.
Smith Woodward's Outlines of Vertebrate Palaeontology (Cambridge, 1898).
On gaining an entry into the blood of a vertebrate the organisms pass rapidly into the general circulation, and are thus carried all over.