Somewhere here I made an analogy between apostolic succession and the two houses of the US Congress. (I had more in a draft, but it seems with the latest edition of this site, the draft option was done away with, along with the drafts). After each election, the House and Senate scrutinize the credentials of each new/re-elected member as to be a member. Some might remember that Obama's replacement in the Senate was questioned, and for a while they would not swear him in. In the House, when an old Congress adjorns, the members vest all their legislative authority as representatives in the Clerk of the House, who, with the new Congress, scrutinizes the credentials of the new members and swears them in, thereby vesting the legislative authority of the House of Representatives in the new members.
In the case of the episcopate, as St. Cyprian noted "the episcopate is one, each one holding the whole for the many." The scrutiny is done by the two/three or more bishops in the surrounding dioceses, with the approval of the primate over all of them, consecrating the new bishop to succeed to the see, and primate remaining in the Orthodox diptychs of the Catholic Church. It does not depend on a bishop consecrating the successor to his see. In fact, his doing so is forbidden by canon and held null and void.
One could try to document the succession of the US Representatives and Senators, but there might be gaps (for instance for the period 1812-1812, burnt in British-Canadian invasion). There are irregularities (the State of West Virginia and its representatives were first seated as for the "true" State government of Virginia at the outbreak of the War Between the States, and was not settled and regularized until after the War ended), but it is amply documented that the Houses have taken their job seriously-indeed, the only real time there is a record to see is when an abnormality occurs and the record comes into being because they correct it.
There is very few if any institutions surviving from Antiquity as well documented as the Orthodox episcopate of the Catholic Church.
We do not have to go back to hoary past to deal with a "gap": in 1700 Patriarch Adrian of Moscow and All the Russias fell asleep, and Abp. Stefan Yavorsky of Ryazan took over as locum tenens, the usual procedure of long, long standing. Abp. Stefan had Apostolic Succession. When the patriarchate itself was replaced by the Holy Governing Synod regime of Bp. Theophan of Pskov in 1721, Abp. Stefan presided over as president until his own departure, when he was replaced by Bp. Theophan. The four remaining Patriarchates of the Pentarchy sent its approval of the Holy Governing Synod at the same time, and the former suffragans, the archbishop of Kolumna, the bishop of Vyatskiy and then the bishop of Tver served the old Patriarchal diocese of Moscow, until 1742, when the Metropolitanate of Moscow again received its own Metropolitan of Moscow with a permanent seat on the Holy Governing Synod, until Metropolitan Tikhon of Moscow was elected as Patriarch of Moscow to replace the Holy Governing Synod in 1917. At his martyrdom, his vicar bishop of Kolumna took over as locum tenens, succeeded by the Metropolitan of Moscow as locum tenens in 1936, who became Patriarch of Moscow and All the Russias in 1943, and there has been regular succession since.
So one can call the years 1700-1917 a "gap," but as far as Apostolic Succession is concerned, they are all accounted for.
