The definition of "Species" gets messed up as soon as you stop reading text books and walk out into the real world.
Here's a conundrum.
A type of seagull common from alaska, through Europe to Asia (following the Arctic Circle)
The Alaskan seagull CANNOT breed with the Siberean Seagull. (Does that make them different species?)
The Alaskan Seagull CAN breed with the European Seagull (Same Species?)
The European Seagull CAN breed with the Siberean Seagull (Same Speices ?) ... ??
Another example.
The Sitka deer (from Japan) is less than half the size of the European Red Deer. The Sitka deer has no antlers/horns; the Red deer has them in spades.
These were always considered to be separate species ... but when a herd of Sitka deer came in contact with a herd of Red deer in the Phoenix Park in Dublin (Ireland), they interbred to the point that some people argue that the Phoenix Park deer represent either a new, true-breeding species, or that the original two species are one and the same.
Typically, the offspring of "related" species within a Genus (between Lions and Tigers; or Ponys and Donkeys) will generate either no offspring, or infertile hybrids.
Even more interesting is that a fertilised mammalian egg can be implanted in virtually any mammalian host and grow to maturity (if there is sufficient room)
Thus cat embryos have been implanted in Rabbits. Dog embryos have been implanted in Cats. In Theory, a human could very easily be implanted in a Cow ! (Just imagine the conversation in the playground ... Your mother's a Cow ... yeah, what's your point ?)
This takes surrogacy to the next conceptual "level", and in the immediate future will put a lot of pressure on governments that currently track "Father" and "Mother" to break that down as
"Genetic Father" - (Sperm Donor)
"Genetic Mother" - (Egg Donor)
"Birth Mother" - (Surrogate or same as Egg Donor)
Guardian(s) - The designated adult(s) that take legal responsibility for decisions relating to the child
Custodian(s) - The designated adult(s) with whom the child lives on a part-timwe or full-time basis
Adoption/fosterage/et al should be legally re-addressed in this light for a very clear moral reason ... every child has the human right (under the UN conventions) to know their true parents, even if they have no contact with them. Additionally, from a medical point of view ... knowing the genetic parents of a person (and their medical histories) makes diagnosis of a variety of medical illnesses vastly easier.
Hmm - off topic. Sorry
Cormac