them. But the mode of simplification adopted in physics and chemistry, viz., experiment, or the arranging of simple artificial conditions, is only to a slight extent applicable to biology. The phenomena are not only complex, but they are so delicately balanced that the introduction of our rude hands in the way of experiment often overthrows the equilibrium, destroys the conditions of biological experiment, viz., life, and thus throws the whole subject into the realm of chemistry and physics. But, fortunately, nature has prepared for us an elaborate series of experiments. We have organisms of every degree of increasing simplicity, from the body of man to the microscopic spherule of almost unorganized protoplasm called a moner. The complex problem of life, as we go down this scale, is made simpler by the successive removal of added complications, until it is finally reduced to its simplest terms, and thus only we begin to understand the essential phenomena—thus only may we find the value of the unknown quantity. It is, therefore, by extensive comparison of organisms in all stages of complication with each other, that the foundations of a scientific biology have been laid. Anatomy has become scientific only through comparative anatomy; physiology through comparative physiology; and, we may add, psychology is now awaiting the development of comparative psychology.
But this general method of comparison is subdivided into three or four sub-methods. Nature has prepared not only one but three or four series, not identical, not mere duplicates of each other, but varied, yet resembling and illustrating each other. The first of these is the natural history or taxonomic series. It consists of the whole series of organisms as they now exist, from the complexly structured mammal to the simple unicelled plant or animal. The second of these is the embrionic or ontogenic series. It consists of the successive stages of development of one of the higher animals, from the germ-cell to the mature condition. The third is the geological or phylogenic series. It consists of the organisms inhabiting the earth in successive epochs, from the Archæan until now. The fourth is the pathological series. It consists of all possible variations from the normal type by monstrosity or by disease. Though much less full than either of the others, it must not be neglected by biologists.
It is wholly by extensive comparison in these four series that biology has recently risen to the rank of a true science. In this great work, the chief credit is due to three men, viz., Cuvier, Agassiz, and Darwin; for these three are the great founders of the comparative method. Cuvier laid the foundations of comparative anatomy and physiology, by comparison in the taxonomic series. Agassiz extended the comparison to the ontogenic and phylogenic series, showed the resemblance between the three, and determined and announced all the formal laws of evolution of the organic kingdom as now recognized. Darwin has made the bold and in large measure successful attempt to explain these laws by the operation of secondary causes. If Agassiz may be called the Kepler