Cloned kittens, phosphorescent bunnies laden with jellyfish genes, children genetically programmed to be smarter, faster, kinder: these are the visions, some real, some imagined, that writer Bill McKibben trots out in his scary new book, "Enough: Staying Human in an Engineered Age."

Many Vermont readers will recognize McKibben as the author of "The End of Nature," a 1989 bestseller that woke its readers to the human and environmental costs of global climate change and other environmental problems. (McKibben, a scholar in residence at Middlebury College, also writes about cross-country skiing for the Free Press.) In "Enough," he tackles the effects of three rapidly advancing technologies: genetic manipulation, robotics and nanotechnology. ("Nanotechnology" refers to machines built on a scale so small that sizes are measured in nanometers, one-billionth of a meter.)

Technology-smitten America is rife with boosters and visionaries who see nothing but good in each one of these new technologies. There's plenty of start-up capital, too. Genetic engineering offers a future where gene therapy, for example, will someday cure illness. But what happens, McKibben asks, when someone alters the genes in a human egg or sperm, a process called germline engineering?

Snip out a disease-causing gene here, splice in a gene for higher IQ there - changes like these made in eggs and sperm will be handed down to each subsequent generation. Encoding changes in the fundamental DNA of a human alters human nature, McKibben argues, by robbing engineered children of free will. A pianist who enhances her daughter's piano playing genes creates a child "ever uncertain whether it is her skill and devotion or her catalogue proteins that move her fingers so nimbly ... (it) robs her forever of the chance to make music her own authentic context - or to choose something else (dance, art, cooking) as the act that brings her to life."

McKibben predicts practical problems along with moral ones. Ever advancing improvements would turn humans into the equivalent of obsolete computers. In such a world, by the time your genetically enhanced child reaches school age, children being born will be that much better engineered - more intelligent, good looking and above average. "They'll be Windows 2050 to his Atari," McKibben says.

Other technologies, such as robotics, also threaten to tear meaning from our lives. If robots improve in handling materials, delivering services, and managing complex tasks, they will some day be in a position to displace virtually all human labor.

That might sound at first like heaven for overworked Americans, but deeper inspection shows it to be a kind of coddled hell, McKibben contends.

Here's one of the more bizarre examples from McKibben's book. Nanorobots, powered by converting biomass to energy and built with the ability to replicate, go haywire (or are released by nanoterrorists) and consume every plant on Earth. It's called the "gray goo" problem. Sounds like a bad Star Trek episode, right? Wrong. The original source publications for this idea - and other equally bizarre ones - can be found listed in the 20-page appendix.

Some of these imaginings are bound to be wrong, but that's not the point. These emerging technologies are complex, and they're advancing rapidly, making it difficult for the average person to understand them, much less debate them. Yet debate them we must, McKibben contends, and in some cases, we'll need to find the wisdom to say "no, thanks."