Page:Popular Science Monthly Volume 39.djvu/605

Rh this process has advanced much further in some places than in others. The general process may be roughly described as concentration of cosmical matter, with dissipation of heat. Along with this go sundry attendant or derivative chemical changes. We find gaseous nebulae; stars ranked in different classes by their colors, perhaps indicating different stages of progress toward consolidation; then planets, first huge ones, like Saturn and Jupiter, with small density, tremendous atmospherical disturbances, and probably some remains of self-luminosity; then such as Mars, Earth, and Venus, with cool, vapor-laden atmospheres and conditions favorable to organic life; then smaller, quickly cooled and solidified globes like our barren moon; then cosmic rubbish like the asteroids and cosmic dust like the meteors. All, of course, are losing heat. Some have cooled too quickly to allow the development of life upon their surfaces; others are still too hot, but while in this stage can perhaps supply radiant heat and actinism for the support of life upon their neighbors. Obviously the gaseous nebula, being a body in an earlier stage of consolidation and containing a maximum of internal motion, is to be regarded as something like what suns and their planets were in a former stage of development.

Long before all these fruits of modern astronomical observation had been gathered, the contemplation of our sun as a consolidating and radiating body had suggested to one of the most profound thinkers that ever lived the famous nebular hypothesis as an account of the mode of development of our planetary system. The nebular hypothesis, set forth by Immanuel Kant in 1755, was the first constructive work toward a definite doctrine of evolution. The theory was restated in 1796 by Laplace, whose line of argument was very similar to Kant's. "Within recent years it has received emendations and qualifications, but the fundamental conception of the nebulous mass acquiring spheroidal shape through rotation, and increasing in oblateness until at some stage in its shrinkage a portion of the equatorial surface is detached as a ring of fragments which ultimately coalesce into a satellite globe—this fundamental conception still remains as a good working hypothesis.

As we now look back over the illustrations here cited—and they are, of course, scanty enough in comparison with what might be adduced—it appears that about half a century ago the foremost minds of the world, with whatever group of phenomena they were occupied, had fallen and were more and more falling into a habit of regarding things not as having originated in the shape in which we now find them, but as having been slowly metamorphosed from some other shape through the agency of forces similar in nature to forces now at work. Whether planets, or mountains, or mollusks,