I don't think that is entirely true. There have been numerous examples of different species of animal that have successfully cross bred.

Leopard (pantheta pardus) and lion (panthera leo)
Horse (Equus caballus) and Zebra (Equus burchellii)
Camel (camelus dromedarius) and llama (Lama glama).... different GENUS and species

Although it is extremely rare from what I have read it is entirely possible

http://members.aol.com/jshartwell/hybrid-mammals.html