not as the rungs of a ladder as Lamarck supposed, but rather as the branches of a tree or a bush; and such branches again like those of a tree bear smaller branches, and these reach to lesser or greater heights from the base level of primitive organization. Thus, anatomy holds that community of plan is an indication of genetic affinities, while modifications of a common plan exhibit the results of adaptation to different ends through evolution. The framework of the human arm is constructed out of the same elements with the same arrangement that we find in the leg of a cat, the flipper of a seal, the paddle of a whale, and even the wing of a bat, different though these structures are in function—and in these resemblances comparative anatomy discerns evidence of a remote common ancestry of men and whales and bats.
Extended through the study of tissues, or histology, to the unitary elements of organic structure—the cells—comparative analysis has brought the whole realm of organic nature under the sway of a great principle—the cell-doctrine of the botanist Schleiden and the zoologist Schwann. This important principle, propounded in 1838 and 1839, produced an immediate effect in unifying organic creatures, though many years passed before it was formulated in the terms employed to-day. In brief, it is this: All the larger organisms are composed of organs which in turn are constructed of various tissues, like muscle and nerve and connective elements; the tissues finally can be resolved into units of structure, the cells, which agree in possessing a central body or nucleus, and in their protoplasmic substance. The elementary nature of cells is still further demonstrated by the simplest organisms we know, which consist of one cell, nothing more and nothing less; while finally the starting point in the development of higher animals is always a single cell—the egg. Truly these are remarkable facts, when we consider the wide range of animal and plant forms.
Vast as the present knowledge is, the tasks of comparative anatomy are not entirely completed. Though voyages of exploration like those of the Beagle with Darwin, the Rattlesnake with Huxley, and above all of the famous Challenger have gone to all parts of the globe, though countless investigators have devoted their lives to the study of special groups like birds and mammals and insects and molluscs, every year brings to light new forms that must be analyzed and placed; while new discoveries in other departments often make it necessary to reexamine known series in the light of fuller knowledge.
While many naturalists prior to the nineteenth century were interested in the way an animal egg produced an adult organism, it was not until the doctrine of descent energized zoology that comparative embryology attained the independent status that it holds to this day. Harvey in 1650 had perceived that, in his own words, "all animals are in some sort produced from eggs." Bonnet and Haller, of the early