A Desert Country Near the Sea: A Natural History of the Cape Region of Baja California, by Ann Zwinger. New York: Harper & Row. 399 pp. $24.95. In the introduction to her new book, ''A Desert Country Near the Sea,'' Zwinger recounts a friend's remark: '' 'It is enough for me to look at a flower and say it is beautiful. For you, you always have to know its name, why it is there, what it is all about.' She was right,'' Zwinger reflects. ''I do need to know who's eating whom and why . . . and why does that cactus grow here and not there . . . and why do the waves sometimes lap and sometimes boom . . . and if far far out beyond my eyesight is there another horizon?''
Whether she is examining the varied contents of a tidepool, clambering up a steep mountainside, or visiting a rancho in the back country, Zwinger notices practically everything. And she is wonderfully prepared to explain the historical, geological, biological, or chemical reasons why. The lyricism of her language captures the movement of nature as drawings cannot, while the logic of her sentences conveys the whys and wherefores beneath the surface of the seen world.
The long, arid stretch of land called Baja (Spanish for ''Lower'') California juts south-southeastward from the state of California (once called Alta or ''Upper'' California). When the essayist and naturalist Joseph Wood Krutch wrote about this part of Mexico in 1961 (''The Forgotten Peninsula''), he found ''a world still what nature rather than human forces have made it.''
Exploring the southern end - the Cape Region - of the forgotten peninsula some 20 years late, Zwinger recalls with nostalgia Krutch's observation about the lack of trash: ''The inhabitants of Baja are too poor to have anything to throw away. . . . And if they acquire something in a can they are very likely to save the container to carry water if it is large enough or to serve as a flower pot if it isn't.'' Yet despite the addition of trash along some roads, Zwinger finds Baja still a place where nature, not man, predominates.
Though Cabo San Lucas and other coastal areas have sprouted hotels and condominiums, the countryside remains much as the earliest European settlers found it. Zwinger quotes the reaction of Jakob Baegert, a Jesuit missionary, who wrote in 1772: ''It seems as if the curse of the Lord, laid upon the earth after the fall of Adam, fell especially hard on California. . . . It is doubtful whether in two thirds of the European continent there are as many prickles and thorns as there are in California alone.'' Bad roads, rough terrain, arid, sandy arroyos, blazing heat, rocky soil, and a scant, increasingly unreliable water supply make the Baja Cape inhospitable, unattractive to developers, but fascinating to naturalists.
Meticulously researched and docu-mented, complete with a chronology and lists of the region's plants and animals, ''A Desert Country Near the Sea'' reads as smoothly as an adventure story. Zwin-ger's learning is a fully integrated, ''natural'' part of the text. Her lyrical flights emerge naturally from her sharp-eyed descriptions, as in this account of a descent from La Laguna, a high-altitude meadow:
''When we are halfway down, clouds of tiny blue butterflies like scraps of sky flutter all around. When the sun goes behind a cloud, they whisk their wings shut and disappear into knife shadows and become old leaves. And when the sun comes out they whirl up again, landing on my pack, my shoulders, my head, rising in helices like blue leaves in a blue wind.''
Zwinger suspects that wilderness may be a luxury ''for those who have the time for it and don't have to fight it continually for living space.'' Yet despite her disclaimer, which is really little more than a recognition of the fact that man, like the rest of nature, cannot help exploiting nature, the thrust of her book, is conservationist. The nature she would preserve is not a mere museum, not a playground, but an ongoing story of geological and biological changes, a history of the precarious adjustments of plant and animal life to the extremities of climate. Measured against the ingenious adaptations of the land snail, the productions of mass civilization have a clumsy, arbitrary look.