The bold line is completely arbitrary, and not exactly a "perhaps" suggestion. It's non-negotiable. Go into any campus biology class and suggest "Perhaps the animals aren't evolving (not saying they dont' "change", but that they didn't descend from a common
ancestor)" - If they think "perhaps the animals did descend from a common ancestor" is a reasonable suggestion, then the opposite is equally reasonable, right? Not really.
There is a lot of speculation in ecumenical terms as to whether the term "day" from the original means "24 hour period of time" as Creationists would have it, or "a period of time - literally one 'cycle'". The creationists are taking the literal stance,
therefore they are taking the Bible literally. I concede that they do not nessisarilly hold the same standards uniformly over the entire text.
The bold line is completely arbitrary, and not exactly a "perhaps" suggestion.
Completely arbitrary - not quite. It's a statement that is:
* Either true or not true.
* Corresponds to a question that is being asked (relating to how species got here)
* The truth value of which provides insight into the question.
It is entirely legitimate to ask the anti-question to that proposition; "Perhaps the animals arn't evolving", however you'll notice that the truth-value does not provide insight into the question how species got here. If
you start from zero-knowledge, then the evidence "that animals are there and they form many distinct species" is true. The question "how/why are there many distinct species, and not just one species, or an infinite spectrum of species" is not answered by the
question "Perhaps the animals arn't evolving", because there is no prior reason to believe that they are evolving.
There are then two perfectly good hypothoses to ask. "What if these species were put there by some higher power (let's call him God)" or an alternative question "What if these species arrived from outer space", or even "What
if these species evolved from a common ancestor (thus allowing us to say that the multitude of distinct species is a special case of there at one point being only one species - a much simpler assumption)". Perhaps I can venture to ask the hypothosis "What
if there was, at one point, infinitely many species, and all the in-between-y ones died out".
All of these are valid hypothoses in science. The next question is to try and systematically discredit the hypothoses by providing counter examples in your set of evidence, and then either
a) Reject the hypothosis because it's just "too wrong".
b) Refine the hypothosis because such-and-such a minor change yields a model for which that evidence does not contradict.
If they think "perhaps the animals did descend from a common ancestor" is a reasonable suggestion, then the opposite is equally reasonable, right? Not really.
The opposite suggestion is perfectly valid - it's just if it's true it doesn't give any insight into why there are many species. The hypothosis "What if all animals were made of sticky-tape and blue-tac" may seem arbitrary,
but if it fits with all the evidence my hypothosis will shed a bucket-load of insight into what and how animals work (or don't). If I ask the anti-question "What if all animals are not made of sticky-tape and blue-tac", and I conclude that so far as all the
evidence can be obtained, animals are in fact, not made of sticky-tape and blue-tac, then this provides no insight. You didn't think animals were made of bits of sticky plastic before, so the sudden realisation that they arn't hasn't changed anything for you,
and your ability to predict things about the world remains unchanged.
That's the reason why the question "perhaps the animals did descend from a common ancestor" was asked and the question "perhaps the animals didn't descend from a common ancestor" wasn't.