An Outline of the History of Restriction on Ijtihad
Among the partisans of the restriction who advanced arguments in its favour is Mawlawi Shah Wali Allah Ahmad ibn `Abd al-Rahim al Dehlawi in his two aforementioned books. Unfortunately his arguments beg the question. His research in both the books contains nothing except claims unbacked by evidence from revelation or reason. The essence of his research is a chronological classification of the generations of Muslims from the time of demise of the Prophet, may peace and God's benedictions be upon him and his Household, to his own time into three categories and an assignment of certain duties to each.
The First Category: This consisted of Muslims who lived during the first and the second centuries. To the laity ignorant of the rules of the Shari'ah among them he assigns the duty of referring, in every issue, to some scholar of the religion, whosoever that may be, who had acquired the ability to deduce these rules, and it was not obligatory upon them to imitate any one particular individual. He observes: "During the first and the second centuries the people were not united in following a specific madhhab. The common people learnt the ahkam from their parents or
the `ulama' of their city who possessed the ability, complete or partial, to infer the ahkam from the sources of the Shari'ah." 
The Second Category: To it belong the generations which lived from after the second up to the beginning of the fourth century. He classifies the people of this category into three classes. The first of these were the full-fledged independent mujtahids, who had authority over three things: first, the freedom to apply the principles of jurisprudence and legal rules; second, to reconcile [conflicting] traditions; and third, to infer detailed rules. 
One who did not have command over all these three was obliged to imitate an independent, mujtahid, irrespective of whether he was an ignorant layman doing taqlid or one belonging to the third class, that of `affiliated mujtahids' (al-mujtahid al-muntasib), that is, one who bases his fatwa on the opinions of one of the independent mujtahids constituting the first class and does not go beyond their opinions. He remarks:
After the first two centuries it was obligatory upon every muqallid and affiliated mujtahid to follow the madhhab of some specific independent mujtahid. 
The Third Category: To it belong the Muslims who lived during the period extending from the beginning of the 4th/10th century to the author's own time. He divides them into two kinds, ignorant laymen and affiliated mujtahids, and says:
It is obligatory upon a layman to imitate an affiliated mujtahid and no other because of the impossibility of there being an independent mujtahid from that time to this day.
He says this after dividing the mujtahids into two classes, the independent and the affiliated, and his claim is that the independent mujtahids became extinct from the beginning of the 4th/10th century and it is not possible for one to exist in view of the non-realization of the above-mentioned qualifications. Hence that which can possibly remain is the second class, that of affliated mujtahids, and it is obligatory for the laity to do their taqlid.
Summarily, it was obligatory for those in the first category to refer to any scholar; for those who belong to the second category to refer only to an independent mu4tahid; and for those belonging to the third category to refer to the affiliated mujtahid, because there exists no other. Then he anticipates a possible objection against this view and goes on to refute it in this manner: