I'm referring to this quotation from Dawkins:
If there were a heaven in which all the animals who ever lived could frolic, we would find an interbreeding continuum between every species and every other. For example I could interbreed with a female who could interbreed with a male who could . . . fill in a few gaps, probably not very many in this case . . . who could interbreed with a chimpanzee. We could construct longer, but still unbroken chains of interbreeding individuals to connect a human with a warthog, a kangaroo, a catfish. This is not a matter of speculative conjecture; it necessarily follows from the fact of evolution.
And I'm putting it together with this recent announcement: Animal/human cybrids aren't working. They don't produce embryos. What this means in practice is that (to some scientists' dismay) animal eggs cannot be used to fix the human egg shortage for all those cloned embryos they want for research. Aw, shucks.