Thoreau often draws positive comparisons between humanity and nature; however, in "Where I Lived, and What I Lived For," Thoreau makes a negative comparison between humans and animals:
Still we live meanly, like ants; though the fable tells us that we were long ago changed into men; like pygmies we fight with cranes; it is error upon error, and clout upon clout, and our best virtue has for its occasion a superfluous and evitable wretchedness. Our life is frittered away by detail. An honest man has hardly need to count more than his ten fingers, or in extreme cases he may add his ten toes, and lump the rest. Simplicity, simplicity, simplicity!
Two similes appear in this passage. The first is "we live meanly, like ants." Ants live by collecting bits of plant and animal matter and bringing them back to an anthill, just as people procure stuff from stores and bring it home. The second simile is "like pygmies we fight with cranes." Pygmies are small birds that fight with other birds. Much like humans who engage in meaningless work, these animals engage in what seem to Thoreau to be fruitless activities.
Note that the structure of these similes within the sentence forms a sort of chiasmus, with the word "like" appearing in the second part of the first simile, whereas the second simile begins with "like." With this clever construction, Thoreau intends to convey his disdain for the "wretchedness" and "superfluity" of urban life. People buy, sell, work, fight, love, build houses, tear them down—and for what, Thoreau wonders? Rather than living a life "frittered away by detail," he commends "simplicity!" People should not waste their lives chasing material comforts; they should rather reconnect with nature and strive for spiritual development. This simile reminds the reader that in order to rise above "mean" and "superfluous" animal status, humans must divest themselves of futile pursuits.
In the Conclusion, Thoreau compares the human spirit to "water in the river," with a simile that captures the powerful force of life that resides in every person:
The life in us is like the water in the river. It may rise this year higher than man has ever known it, and flood the parched uplands; even this may be the eventful year, which will drown out all our muskrats. It was not always dry land where we dwell. I see far inland the banks which the stream anciently washed, before science began to record its freshets.
The passage begins with an extended simile that compares the life of human beings to the water in a river. Thoreau then claims that the land was not always "parched" and goes on to describe a river that "may rise [...] higher than man has ever known." In other words, Thoreau mourns modern society as a spiritual and intellectual desert, but remains hopeful that people will recognize the great life force within them. The phrases "the life in us" and "where we dwell" unify humanity (both within itself and with nature). Eventually, everyone should be able to rise to the challenge of navigating the modern world.
This simile helps to neatly conclude the piece by renewing Thoreau's optimism and confidence that the human spirit will prevail and come into closer contact with nature through a simpler way of life. Despite the aridity of the modern world, each individual has the potential to return to his powerful primal state. The image of a flowing river contrasts with the stable, static image of the pond, and it serves as a reminder of the ever-changing nature of humanity as well as the raw untapped power in each and every person.