Marianne Moore structured much of her poetry around observations of the visible, most of which she absorbed from the books she knew and loved so well — she did not travel much, and in her work exhibits more comfort with the natural or animal world than with that of her own humankind. Her early study of biology and her work in a library as a young woman combine in her poetry to a nearly clinical effect, what amounts to catalogic treatment of her subjects. She forbids herself access to what must have seemed a selfish and easy indulgence, the kind of romantic, maudlin sentimentalism on which the lyric mode might otherwise depend. Of her poem “In Distrust of Merits” , a fine expression of an anti-war stance, Moore says:
I do like it; it is sincere but I wouldn’t call it a poem. It’s truthful; it is testimony — to the fact that war is intolerable, and unjust…[but it is] haphazard; as form, what has it? It is just a protest — disjointed, exclamatory. Emotion overpowered me. 
Even still, Moore’s works do, perhaps despite their author’s will, betray the joys and the frustrations of an active emotional life, which the diligent reader will find buried in the encyclopedic descriptions that encode and enwrap a precise imagination.
Moore writes in “He ‘Digesteth Harde Yron’”:
The power of the visible / is the invisible. (49-50)
The direct reference is to the continued physical existence of the ostrich, the bird’s hardiness captured in the very title of the poem itself; the implication is that life is only as meaningful as that which it endures. The ostrich has survived exploitation at the hands of men in search of splendid plumage for decoration; of great eggs and flesh to eat; of novelty and curiosities for display; and even for the strong back of a beast to ride. As a means of contrast, in order to qualify and support the fact of the ostrich’s perseverance, Moore mentions the names of some other large birds who lacked the main defensive gesture of most birds — flight — and which went the way of the dodo as a result. Moore’s claim that the ostrich
was and is / a symbol of justice” (6-7),
a significance that has spanned since the time of Xenophon, derives from its unlikely endurance against the weighted odds of wanton human devastation. Although she does not do so explicitly, Moore’s diction at the poem’s close belies her great respect for the ostrich:
an alert gargantuan,
little-winged, magnificently speedy running-bird.
This one remaining rebel
is the sparrow-camel. (59-63)
When Moore writes about the degrees of difference between things of a similar nature, between large flightless birds for instance, she points out the way each instance of life is most itself as kind of divergence from its similars. In “Digesteth,” Moore underscores the absences of the aepyornis, the roc, the moa and the auk, the ostrich “linked / with them in size” (4-5) and in flightlessness, through an implementation of apophasis, a process of description by negation; the negative presences of the other great birds serve to make that much more meaningful the survival of the ostrich. It is by observation of some special trait, some divergence, that one can determine the measure or unique quality of a thing as it pertains only to itself. Moore’s purpose in another poem, “The Buffalo,” is not to recite the relational ties between a buffalo and various kinds of cows or oxen; instead she imbues her subject with importance via the negative space created through apophatic, or negatory, construction. From the extinct Aurochs to the domesticated cows and oxen of contemporary agriculture — Brown Swiss, Hereford, Holstein, Vermont ox or even bison — none but
buffalo has met
human notions best. (24-6)
Once again, Moore defines a desirable quality by its defiance of expectation, as “the Indian buffalo…need not fear comparison with bison…indeed with any of ox ancestry.” (55-61)
Moore’s interest in the strange or unfamiliar fact is a representative fascination common to the poets and artists of her generation, who, bored or unchallenged by the forms of the not-so-distant past, write to find new paradigms, and to forge new molds. It is not only the genetic fitness, of the ostrich or buffalo for instance, but more often examples of superlative mutation that find their fantastically weird ways into Moore’s good graces. For her, it is the zig in the line and not the traced ruler-edge that deserves comment and praise, as in her poem about a mutated camperdown elm, or the peach pit that grows nectarines. In order to highlight the spectacular, Moore chose to veer from the poetry of the personal and to forsake the sentiment of individual experience. This impulse toward a conspicuously absent ego compels her to write human presence out of primacy, as in “The Octopus,” a poem, in short, about the immense grace of a glacier. Humanity co-exists with the icy mountain only in the traces of its arrogant confiscation of perspective, as in the description of a body of water as:
that lady-fingerlike depression in the shape of the left human foot
or in the sight of:
thoughtful beavers making drains which seem the work of careful men with shovels.
In contrast to that projection of potency and mastery, are the inscriptions made in the steeps of the mountain by human hands, unseen and minute
which stipulate ‘names and addresses of persons to notify
in case of disaster.’
 Moore, Marianne. Collected Poems. New York: MacMillan, Second Printing, 1952.
 Hall, Donald. “The Art of Poetry No. 4: Marianne Moore.” The Paris Review Issue 26, Summer-Fall 1961.