Theories of the Earth's Origin — Upham. 207 
nebiilEe is the presence of two dominant arms that arise from 
diametrically opposite sides of the nucleus and curve concentric- 
ally away. No single-arm spiral of the watchspring type has been 
found, so far as I am aware. There are often more than two 
arms in the outer part, and there is much irregularly dispersed 
matter, but even in the more scattered forms the dominance of 
two arms is discernible. 
A second feature of note is the presence of numerous nebulous 
knots or partial concentrations on the arms and more or less out- 
side them. So, also, the more diffuse nebulous matter is un- 
equally distributed, and in some of the forms, regarded as young- 
est, dark spots and lines emphasize the irregularity. 
All these features go to show that these forms are controlled, 
not by the support of part on part, as in a continuous body or in 
a mass of gas or even in a definite swarm of quasi-gaseous me- 
teorites, but by some system of combined kinetic energy and 
gravity which permits independence of parts. It is, therefore, 
conceived that the innumerable solid or liquid particles which 
the continuous spectrum implies revolve about the common cen- 
ter of gravity as though they were planetoidal bodies. If this 
were certainly known to be the case, these might well be called 
planetesimal nebulae. 
It is clear from the tenuity of these nebulae, as seen from 
the side of the spiral, that they are disk-like, and this is directly 
shown to be so when they are seen obliquely. In their disk-like 
shape, these nebulae conform to the mode of distribution of mat- 
ter in the solar system. Within the area of their disks, also, the 
distribution is irregular, as it is in .the solar system — a fact too 
much overlooked by reason of our predilection for symmetry, un- 
der the influence of the symmetrical Laplacian conception. 
All of the more familiar spiral nebulae have dimensions that 
vastly transcend those of the solar system, and they cannot be 
taken as precise examples of the solar evolution. * * * It is 
to be hoped, however, that the present rapid progress in the per- 
fection of instruments and of skill will soon bring within the 
reach of successful study some of the smaller spiral nebulae that 
represent the solar system more nearly in mass and proportions. 
With this much of knowledge and of limitation of knowledge 
relative to existing nebulae, the construction of a working hypo- 
thesis required not a little resort to supplementary deductive 
and hypothetical considerations The inference that a spiral 
nebula is formed by a combined outward and rotatory movement 
implies a pre-existing body that embraced the whole mass. In 
harmony with this, an ancestral solar system has been postulated 
— a system perhaps in no very essential respect different from 
the present one. * * * 
To this conception of an ancestral sun with an undefined 
antecedant history as a star, question will arise at once as to a 
