It is an interesting zoological fact that animals don’t sexually desire animals outside their own species (excluding pathological cases). A particular species may not strike us as sexually desirable, but within it we may suppose that sexual desire runs high; even the most beautiful species on earth will not attract members from outside its ranks. The loveliest human female has nothing to fear from the lascivious attentions of orangutans, say. Sexual desire is species-centric. Why is this? Is it some sort of innate sense of propriety or an instilled taboo? The basic equipment for copulation is present, but the interest is lacking. The reason, as any biologist will tell you, is that species cannot interbreed, so cross-species sex will never lead to offspring. If a gene arose by chance that encouraged sex across species boundaries, it would never make it into future generations, since the mechanics of reproduction disallow the requisite mixing of DNA. Perhaps such a gene has arisen many times, but each time it quickly exits the gene pool because it cannot lead to offspring in which it recurs. If things were different, if species could interbreed, then we may suppose that sexual attraction would be far more promiscuous, since a gene for cross-species sex would lead to offspring containing that gene. Animals would favor animals that look (and smell) quite unlike them—mammals desiring reptiles, birds fancying moles. The result of such copulations would be strange (to us) hybrids, the fruit of inter-specific sex. What prevents this from being the case is a basic physical fact, viz. that DNA can’t combine if it is too different from other DNA. Species sexual psychology is the outcome of a genetic law at a basic chemical level—a kind of psychophysical law. The physics shapes and constrains the psychology. That is the sole explanation for why animals don’t breed with other species. Dogs can breed with other breeds, given the genetic facts, and dog breeds vary enormously; hence dog sexual psychology is flexible as to phenotype. That would be the model for all sexual behavior if DNA were more accommodating. Humans, say, might have a thing for big cats or even elephants, if the DNA were sufficiently flexible: emotion would follow chemistry.
And it is not just a matter of glandular excitation; there is also the question of rivals. Each animal of a given species competes with other animals for mates, sometimes fiercely, even fatally. Males fight other males for access to females. But they don’t worry about males of other species, because these individuals cannot impregnate females of their own species and have no wish to do so. The psychology of aggression and competition is limited to members of the animal’s own species—other orangutans, say. The reason again is that the DNA of other species cannot combine with the DNA of the species in question, so there is no genetic need to ward off suitors from other species. It would be quite different if this were not so: then an animal would need to compete across species boundaries. A male squirrel, say, may need to fight off a handsome snake for the attention of female squirrels; human males may find themselves pitted against massive orangutans for the attentions of human females (the fights would be terrible to behold). The psychology of male-male interactions would conform to the genetic possibilities afforded by inter-species sexual congress. Again, the psychology would be fixed by the chemistry of DNA—not by any supposed proprieties or taboos.
Perhaps on other planets life has evolved without the constraints that terrestrial DNA labors under; on these planets inter-specific sex is common and deemed entirely normal. It would be thought strange to limit one’s erotic interest to one’s own species (too many fish in the sea, as it were). No doubt our own psychology (and that of other terrestrial species) would recoil at such freewheeling ways, but the logic of Darwinian genes predicts it. There is nothing in standard Darwinian evolutionary theory to preclude inter-specific sex, merely the chemical possibilities inherent in DNA. Animals will mate with any animal that can carry its genes into the next generation; species identity is irrelevant. Species psychology simply tags along with the genetic story. If our DNA were less choosy, so would we be.
It may be objected that the scenarios I have envisaged are not logically (metaphysically) possible for the simple reason that interbreeding is the criterion of species identity. If two animals can reproduce together, then they must be of the same species, by definition. But this objection is misguided even if we accept the proposed criterion of species identity. Let us suppose that successful copulation between an orangutan and a crocodile (say) does entail that the two animals are of the same species; that does not gainsay the point that enormous phenotypic differences are compatible with sexual desire. If such a thing became possible tomorrow, there would still be a striking disconnect between psychology and genetics—the animals may as well belong to different species so far as phenotype is concerned. Besides, the interbreeding criterion only works because of de facto correlations between genetic possibility and phenotypic resemblance: if cold-blooded quadrupeds could interbreed with warm-blooded bipeds, we would still need a way to mark the phenotypic distinction—so we may as well say they belong to different species that happen to be able to produce offspring.
No doubt it seems entirely natural to each species to be attracted only to members of its own species—as if anything else is inconceivable—but in reality sexual psychology is fixed by chemical facts that lie outside of anything psychological, or even physiological. Let alone anything preordained by an approving or disapproving divine creator. Intra-specific sexual attraction is an artifact of chemical combinability. Sexuality would be quite different in a world in which DNA could more freely intermingle.
 The perceptiveness of animals is markedly superior within their own species, as things stand. The reason for this is that animals need to evaluate potential mates or rivals with greater acuity than members of other species given that their genetic future depends on it. In a world in which cross-species reproduction occurs, however, animals would need to extend their perceptiveness beyond their own species; they would need to perform the same exact calculations and careful appraisals of members of other species in order to maximize gene propagation.