If it were to be established by some future investigations that animals differ from men only in degree and not radically in kind, we would then no longer have any moral basis for treating them differently from men, and, conversely, this knowledge would destroy our moral basis for holding that all men have basic rights and an individual dignity.This seems a nice expression of an idea I've heard and read by others as well. But it doesn't obviously make any sense. Let's consider the two claims it makes separately:
(a) If animals differ from humans only in degree and not radically in kind, then we have no moral basis for treating them differently from humans.
I think there is one sense in which this is true, and while that is presumably terrifying to Adler, it's not a new point. If animals are like humans in ways which, when thinking just about humans, we regard as morally important, then animal lives have similar moral importance. At minimum, that just means we can't treat them as if they are "mere things." And given the social and emotional complexity of many animals, it may mean quite a bit more, morally.
But there's also a sense in which this needn't be true. It certainly doesn't follow that we should treat animals in exactly the same ways we treat humans, and for roughly the same sorts of reasons that we shouldn't treat children in exactly the same ways as we treat adults, or pupils in exactly the same way we treat intellectual peers, and so on. (Edit: I didn't mean to imply anything about paternalism by choosing those examples, but rather that different cases often require different treatment without the difference being inappropriate or unfair.) Another point to be made here is that we may well have some reasons to think that our responsibilities to humans are stronger, in general, than our responsibilities to animals, on the model of the special responsibilities parents have to their own children. But this wouldn't imply that other animals (or other children) make no demands, or that special interests imply that anything goes in the cases which don't involve them (or that special interests always trump other interests, etc.).
(b) If animals differ from humans only in degree and not radically in kind, then there is no moral basis for holding that all humans have basic rights and an individual dignity.
I simply can't see why the consequent of this has any connection to the antecedent. It would follow just as well from the antecedent that animals will have some share of moral rights (depending perhaps on the degree of relevant similarity) and of individual dignity. To state my point formally, I think this is alarmist bullshit, and that the alarm gets sounded because it's so obvious that many animals are treated terribly by systems and institutions from which we derive great comfort and convenience. Because of this, acknowledging the idea of animal dignity would force (rationally speaking) a re-examination of, and in many cases the rejection of, those systems and institutions. (And here, claims about rights need be only pragmatic, not metaphysical.)
I have NO IDEA why the existence of "animal dignity" would impugn the existence of "human dignity." Of course, if animals differ from us only in degree rather than kind, then that means a certain creationist story about the specialness of humans is false. (It also means that a Cartesian story about our specialness as reasoners and language-users is false, or at least misleading.) "Dignity" just means something like having an inherent worth, and comes with overtones of being an end rather than a means, of being an irreplaceable individual, of being an individual which, as Rhees might say, is "something that can be loved." Why should the possession of that by animals mean that humans can't still have their own dignity, too? In short, it doesn't. What it does mean is that we have to be suspicious of the business--which we can roughly blame on Kant--of suggesting that only human beings warrant respect in themselves.