The Romans claimed that the senate had its origins in the patres, the hundred advisors to Romulus. Tradition maintained that these hundred men were the heads of families--gentes--and were also the origin of the patrician order. The senate, increased to three hundred members early in the Republic, was never intended to be representative of any particular aristocratic body, at least not in a systematic way. Instead, membership in the senate was granted, by various means, for what was perceived as outstanding service to the state. Usually, by the middle of the Republic or so, that meant the quaestorship or some equivalent magistracy or service. By the late Republic the quaestorship had become an explicit requirement for senatorial membership--upon ending one's term as quaestor one was automatically coopted into the senate. As a result, election to magistracy was always the most important requirement for senatorial membership in the historical period. Now, while novi homines did exist the senate was always dominated by the nobiles, those who had senatorial ancestors (or consular ancestors, to use an alternate definition). Nevertheless, there was neither any limit on the number of individuals from a particular gens that could hold the same office at the same time, nor was there any such provision for senatorial membership. Nor would it make sense: senatorial clans could be enormous, such as the gens Cornelia which across patrician and plebeian branches was possibly the largest clan in Rome. Cicero and his brother, both novi homines, were part of the senate in the 60s and 50s. Caesar and his cousin Lucius (cos. 64, the year before Cicero's consulship) both were sitting in the senate in 63 when Caesar famously proposed that the Catilinarians not be executed without trial. In the second half of the second century, during what Syme called the "Metellan age," members of the Caecilii Metelli nearly monopolized the elected magistracies, and correspondingly maintained a large number of spots in the senate. By the same line of reasoning, due to the way that senatorial membership was conferred, ancestry alone was not sufficient cause to be offered enrollment into the senate. Though family was extremely important (Cicero accused L. Piso, of the illustrious plebeian noble gens Calpurnia, of abusing his name to win election and then failing to live up to his senatorial ancestors' memory) election or (prior to the late Republic) notable action was required--moreover, the senate's numbers were restricted, although not all that systematically. Fairly large numbers of patrician and plebeian senatorial nobility failed to win election in their lifetimes, or were uninterested. These, along with senatorial hopefuls still too young for office, made up a known part of the equites, and attached themselves often to known politicians for a time--Antony supposedly was attached to P. Clodius in his youth, and Keaveney supposes that these senatorial hopefuls were the main part of the "anti-senate" that Plutarch says Sulpicius Rufus mustered. Moreover, senatorial families died out, at least in the senatorial line. This phenomenon is most noticeable in the patrician nobility rather than the plebeian, because of the ease with which patrician status was lost to posterity, which itself helped reduce the birthrate of patrician senatorial clans, by preventing them from passing on their status in the event of marriage into senatorial or equestrian plebeian families. By Caesar's time plebeian nobiles dominated the senate. The OCD estimates that of about fifty or so patrician families known from the early Republic (which itself puts the hundred patres under suspicion) only fourteen still had surviving patrician lines. Of those that maintained plebeian lines, many were no longer political. The patrician gens Quinctia, which spawned the famous dictator, held a number of consulships in the fifth and even fourth centuries, as well as sporadic office in the third and occasionally the second centuries. From the middle of the second century the family all but disappears, resurfacing through the plebeian line in the senate only with L. Quinctius, tribune 74 and praetor 68. Shortly thereafter the family disappears politically--the last Quinctius I know of was suffect consul in 69 AD. The (also patrician) gens Sergia, which spawned Catiline, was notably unimportant during pretty much the entire historical period. Though they held a couple important consulships early in the Republic, following this there is only a single known senatorial Sergius (M. Sergius Silus, urban praetor 197, who famously lost his right hand fighting the Carthaginians) before Catiline. No doubt Sergii were admitted at least sometimes into the senate during the intervening period, but it's highly likely that quite a lot of Sergii never attained election to even the quaestorship, and thereby admission into the senate. And no wonder: there simply were not enough magistracies for everyone to prove by election their worthiness for admission to the senate. Curule magistrates were coopted almost always after their year of office, but there were so few of these. Plebeian aediles weren't enrolled until 123. While quaestorship could often lead (though hardly always) to entry into the senate before its formal establishment as a requirement, before Sulla there were simply too few quaestors for everyone to win election--only ten, following 267. A decent number of political failures must have existed in the senatorial class in any given generation.