THEODORE ROOSEVELT was born in 1858 and died in 1919. So we can think of the Roosevelt era as running from the Civil War through World War I. David McCullough's previous books (on the Johnstown Flood, on the Brooklyn Bridge, and The Path Between the Seas , on the Panama Canal) have dealt with large events of that era. He sees TR as another kind of wonder; and Roosevelt's well-known isthmian hustling was indeed once summed up in a neat palindrome -- "A man, a plan, a canal, Panama."

McCullough has thus arrived at TR biography by a logical progression. He explains that his historian's interest has centered on acts of creation or formation. His prticular concern accordingly is with the evolution of TR's character during adolescence and early manhood. The story of Mornings on Horseback (a title of no particular significance, just a nice phrase) close in 1886, by which point Roosevelt was set on course In 1886 TR came back East from his interlude as a Dakota ranchman, ran enterprisingly through unsuccessfully in New York City's mayoral election, and got married for the second and final time.

Such an approach seems sound. "The child is father to the man," in Wordsworth's aphorism; and Erik Erikson is among the psycho-historians who argue the importance of the individual identity crisis associated with teen-age life. TR in fact almost demands treatment of this sort. Comment on him invariably emphasizes the notion that he is young in heart. Admirers call him "boyish" or "youthful," critics condemn him as "immature" and even "infantile." In one view he is active, a constructive doer; in the other TR is restless, frenetic, a destructive brat.

David McCullough brings considerable gifts to the task of explicating TR. The "Teedie" of the family nickname porvides rich material for historical psychologizing. He adores his parents and his elder brother Elliott (the father of Eleanor Roosevelt), and is adored by his two sisters as well as by his mother and father. His mother is a Southern belle, whose brothers fight resourcefully for the Confederacy. His father Theodore Sr. -- rich, intelligent, warmhearted -- hires a substitute; for him the Civil War is a civilian war. Teedie is a puny child, educated by tutors until he becomes a freshman at Harvard. His thoughtful, dominant father dies while TR is still an undergraduate. TR can start his junior year by dropping the ignominious "Jr." from his name. Soon he is the recipient of an income well above that of Harvard's President Charles Eliot, and spending his inherited wealth like a millionaire dandy. Young Roosevelt, clubman, socialite, athlete, likewise loses no time in getting engaged. He is married at 21, to a Boston heiress. Horribly and unexpectedly, his wife and mother die on the same day, of unrelated causes.

All of this is facinating. Some of McCullough's best analysis concerns TR's asthma. So far as I know, he is the first to notice that TR's childhood attacks most often occurred on Sunday, the day when the Roosevelts followed the conventions of the era, confining themselves to religious attendance and to decorous dullness. McCullough has spoted a plausible explanation: As a small boy, Teedie was terrified of the neighborhood church. He confessed to his mother his belief that the building contained a dangerous animal called a "zeal," which he had heard of in a reading from John 2:17 ("The zeal of thine house hath eaten me up"). McCullough notes a possible link with Teddie's juvenile (and of course lifelong) passion for natural history. At the age of 7 or 8 TR was obsessed with a dead seal which he saw laid out in a fish market on Broadway. "Zeal" and "seal" sound nearly the same.

A problem for the author is that he is not the only one to be intrigued by such bully material. Carleton Putnam's Theodore Roosevelt: The Formative Years , 1858-1886 (1958) covers the same period with impressive competence. More recently, Edmund Morris' much-praised The Rise of Theodore Roosevelt (1979) takes TR up to 1901, when he succeeded the assassinated President McKinley. More than 300 pages of Morris -- a very long book -- are devoted to the years before 1886. Putnam's volume is perhaps not in contention, in that it exemplifies the attitudes of an earlier generation. Putnam for example make almost no mention of the growing instability of brother Elliott, womanizer and drinker, who was to die disgraced in 1894. The contest if any is between Morris and McCullough, both of whom deal candidly though not vulgarly with family relationships.

Both books will probably find an audience. Possibly in an effort to differentiate Mornings on Horseback from the Morris biography, McCullough insists that he is in fact not writing about TR alone but about the whole family. The mass of Roosevelt letters and diaries in Harvard's Houghton Liberty allowed him, he says, "the chance to get inside the life of a well-to-do Victorian American family . . . in a way that is seldom possible for a writer, except in fiction." This raises expectations of a hoard comparable to that of, say, the James family -- as in Jean Strouse's subtle new study of Alice James.

The result is not quite up to that standard. No doubt the fault lies mainly with the Roosevelts. They are a close, affectionate tribe. They are copious correspondents. But in the main they appear decently conventional, expressing themselves in the hearty, usually unrevealing commonplaces of upperclass Anglo-American. Their jovial camouflage may have served to discourage McCullough from persisting in the kind of speculation that illuminates his treatment of TR's asthma. Why for instance did Elliott, the handsome eldest sibling, wreck himself? Why was Corinne so relutant to marry? How much did TR disapprove of his father's decision not to enlist in the Union forces? Was the family consciously perturbed by its Union-Confederate doubleness? TR's asthma left him as he neared manhood: Why did it come back again? Did he blame himself for the death of his wife Alice -- more or less a death in childbirth? Such questions are touched upon, but they are not often probed; and Theodore Roosevelt Jr. remains the central rider in the cavalcade.