has continued in the march of development, while the career of the other has been stopped at an earlier point. The organic aspect, at last assumed, is the representation of the physical agencies that have been at work—the environment. Had these for any reason varied, that variation would at once have been expressed in the resulting form, which is, therefore, actually a geometrical embodiment of the antecedent physical conditions. For what reason is an offspring like its parent, except that it has been exposed during development to the same conditions as was its parent. Comparative physiology is not a fortuitous collection of experiments. Our noblest conception of it is the conception we have of analytical geometry. Each member of the organic series is an embodied result of a discussion of the equation of life for one special case.
It was a felicitous thought of Descartes that we may represent a geometrical form in an algebraic equation, and, by the proper consideration and discussion of such an expression, determine and delineate all the peculiarities of such a form; that here it should become concave, there convex; here it should run out to infinity, there have a cusp. The equation determines all the peculiarities of the form, and enables us to construct it. In like manner, all living and lifeless forms are related; an increase in the value of one condition carries development forward in one direction; an increase in the value of another condition determines development in another way, and these variations give rise in their succession to the whole organic series.
Nature ever geometrizes and ever materializes. Every organism is the result of the development of a vesicle, under given conclusions, carried out into material execution. It is the incarnation, the embodiment, the lasting register of physical influences, the daughter of the environment.
Let us now rapidly survey the changes that have taken place in the earth's organisms:
In the earliest, or Primordial period, there existed of plants only water-organisms—tangled sea-weeds. Then in the following, the Primary, came the more perfect cryptogams, such as ferns. Then followed, in the Secondary, pine-forests. In the Coal period the phanerogamia developed out of the more perfect cryptogamia. Not until the Chalk did the higher corollifloræ appear. In the beginning of the Tertiary the earth had sufficiently cooled at the poles, climate-zones were produced, and the land was covered with leaved forests. Flowerless plants had been succeeded by flowering ones, the latter first without a distinct corolla, and then by those with one; and of these, first the lower and then the higher.
Turning to the order of succession of animal life—of the Primordial, the forms are skull-less; then in the following, the Primary, came fishes, first those with the heterocercal tail, as in the embryos of