His study is a big square room on the top floor of one of the college buildings; and in this room is a bookcase extending from ceiling to floor, given up to his own works.
Copies of every edition and of all translations are here.
And in a special case are the original manuscripts, solidly bound in boards, as carefully preserved as were the "literary remains" of William Morris, guarded with the instincts of a bibliophile.
Of the size of this Haeckel collection one can make a guess when it is stated that the man has written and published over fifty different books. These vary in size from simple lectures to volumes of a thousand pages. His work entitled, "The Natural History of Creation," has been translated into twelve languages, and has gone through fifteen editions in Germany, and about half as many in England.
The last book issued by Professor Haeckel was that intensely interesting essay, "The Riddle of the Universe," which was written in Eighteen Hundred Ninety-nine, in two months' time, during his summer vacation. He gave it out that he had gone to Italy, denied himself to all visitors who knew that he had not, and answered no letters. He reached his study every morning at six o'clock and locked himself in, and there he remained until eight o'clock at night. At noon one of his children brought him his lunch.
Unlike Herbert Spencer, whose later writings were all dictated--and very slowly and painstakingly at that--Haeckel writes with his own hand, and when the fit is on, he turns off manuscript at the rate of from two to four thousand words a day. In writing "The Riddle of the Universe," he took no exercise save to go up on the roof, breathing deeply and pounding his chest, varying the pounding by reaching his arms above his head and stretching. However, after a few weeks the villagers and visitors got to looking for him with opera-glasses; and he ceased going on the roof, taking his calisthenics at the open window.
This exercise of reaching and stretching until you lift yourself on tiptoe, he goes out of his way to recommend in his book on "Development," wherein he says, "There is a tendency as the years pass for the internal organs to drop, but the individual who will daily go through the motion of reaching for fruit on limbs of trees that are above his head, standing on tiptoe and slowly stretching up and up, occasionally throwing his head back and looking straight up, will of necessity breathe deeply, exercise the diaphragm, and I believe in most cases will ward off diseases and keep old age awaiting for long."