The extension of the principle of the reflex to include behavior involving more and more of the organism was made only in the face of vigorous opposition.
My understanding is this: the vigorous opposition is the reason why the principle of the reflex extend to include behavior involving more and more of the organism. But I'm not so sure about it, the word only in here seems to say vigorous opposition is the only reason, that doesn't make much sense.
The source is Skinner's "Science and human behavior" and following is the whole paragraph:
The extension of the principle of the reflex to include behavior involving more and more of the organism was made only in the face of vigorous opposition. The reflex nature of the spinal animal was challenged by proponents of a "spinal will." The evidence they offered in support of a residual inner cause consisted of behavior which apparently could not be explained wholly in terms of stimuli. When higher parts of the nervous system were added, and when the principle was eventually extended to the intact organism, the same pattern of resistance was followed. But arguments for spontaneity, and for the explanatory entities which spontaneity seems to demand, are of such form that they must retreat before the accumulating facts. Spontaneity is negative evidence; it points to the weakness of a current scientific explanation, but does not in itself prove an alternative version. By its very nature, spontaneity must yield ground as a scientific analysis is able to advance. As more and more of the behavior of the organism has come to be explained in terms of stimuli, the territory held by inner explanations has been reduced. The "will" has retreated up the spinal cord, through the lower and then the higher parts of the brain, and finally, with the conditioned reflex, has escaped through the front of the head. At each stage, some part of the control of the organism has passed from a hypothetical inner entity to the external environment
Is my understanding correct?