"Among American writers Thoreau was the pioneer of nature-study. Audubon had preceded him but he worked mainly with the brush; to multitudes Thoreau opened the gate to the secrets of our natural environment. The subtle delicacy of the grass-blade, the crystals of the snowflake, the icicle, the marvel of the weird lines traced by the flocks of wild geese athwart the heavens as they migrated, these he watched and recorded with loving accuracy and sensitive poetic feeling as no one in our land before had done. I have thrown a stone upon the cairn at Walden Pond which has now grown so high through the tributes of his grateful admirers. I shall throw still others in grateful admiration if the opportunity comes to me."