In both these examples, the events mentioned in the verbs are future; "if any member shall absent himself;" "if thou shalt neglect;" therefore the auxiliary verb shall should have been employed, or the sentences should have been elliptical, "if any member absent himself;" "if thou neglect;" where shall is understood and easily supplied by the reader.
Numberless examples of the same kind of inaccuracy may be found in good authors. Thus in Haley's Happy Prescription, act 2.
"And if my scheme prospers, with joy I'll confess,
What a whimsical trifle produced our success."
The idea is, "if my scheme shall prosper;" and this is obvious by the subsequent part of the sentence, where the future is employed, "with joy I'll confess."
"If Punch grows extravagant, I shall reprimand him very freely; if the stage becomes a nursery of folly and impertinence, I shall not be afraid to animadvert upon it."——Spect. No. 35.
These should have been grow or should grow; become or should become.
"If any thing offers (shall offer) from Dublin, that may serve either to satisfy or divert you, I will not fail," &c.—— Swift's Corresp. letter 2.
In the following passage, the same writer is much more correct.
"If any one matter in it prove (that is, shall prove) false, what do you think will become of the paper?"——Letter 8.
But the use of the future for the present is much more frequent.