For example, we are told that some of the muscular movements involved in the instincts--such, for example, as the bird's nest-building--are so complex and so finely adjusted to an end, that it is straining belief to suppose that they could have arisen gradually by reflex adaptation alone.

There is also a further difficulty with the reflex theory which has seemed insurmountable to many of the ablest psychologists of animal life; the difficulty, namely, that many of the instincts require the action of a great many muscles at the same time, so acting in "correlation" with or support of one another that it is impossible to suppose that the instinct has been acquired gradually.

For in the very nature of these cases we can not suppose the instinct to have ever been imperfect, seeing that the partial instinct which would have preceded the perfect performance for some generations would have been not only of no use to the creature, but in many cases positively injurious.

For instance, what use to an animal to be able partly to make the movements of swimming, or to the birds to build an inadequate nest?
Such instincts would not be usable at all.

So we are told by the second theory that the animals must have had intelligence to do these things when they first acquired them.