In other words, we are here dealing solely with the restriction imposed by the 16th Amendment on the right to resort to the source whence an income is derived in a case where there is power to tax for the purpose of taking the income tax out of the class of indirect, to which it generically belongs, and putting it in the class of direct, to which it would not otherwise belong, in order to subject it to the regulation of apportionment.
You forgot to mention that the tax discussed in this statement is upon real or personal property which is what is subject to apportionment at the time of Pollock, not upon the activities of individuals engaged in trades vocations or employments which are subject to excises under Article I Section 8 of the Constitution.
OOPs seems as you overlooked quite abit there.