THE ORIGIN OF THE EARTH. 13 



very greatly by photography, nebulae have not been found which 

 present a series of such systematic annulations as the hypothesis pos- 

 tulates. On the contrary, the spiral nebula, as announced by the 

 late Professor Keeler, 1 is found to be the dominant type. 



II. The Meteoritic Hypothesis. 



A simple but untenable form. — It was long ago noted that, from 

 time to time, fragments of stony and metallic matter fell from the 

 heavens, and that shooting stars entered the upper atmosphere nightly 

 in great numbers; and out of this naturally grew the suggestion that 

 the earth may have been built up in this way, save that the process 

 was rapid in the early ages before the heavens had been so thoroughly 

 swept of meteoritic material. This form of the hypothesis, however 

 simple and natural, may be dismissed without serious consideration, 

 for the distribution of meteorites, their directions of motion, and their 

 velocities are such as to forbid the belief that the solar system, with 

 its symmetrical discoid form and its many peculiar and significant 

 features, could have been formed from them directly in the manner 

 supposed. 



The hypothesis of Lockyer and Darwin. — In a work entitled "The 

 Meteoritic Hypothesis " Lockyer has endeavored to show that nebulae 

 are composed of meteorites sparsely aggregated into swarms, and that 

 stellar systems are evolved from them. His hypothesis is, therefore, 

 nebulo-meteoritic, and relates to a stage antecedent to the formation 

 of the planets. He assigns the light of the nebulae to the collision of 

 the meteorites with one another. To meet the fact that the spectra 

 of some nebulae are of the gaseous type, he assumes that the impact 

 vaporizes a part of the meteorites, and these vaporized portions give 

 forth the gaseous spectra. It remains to be explained, however, why 

 the spectra of these nebulae rarely show anything but hydrogen, helium, 

 and an unknown substance, or substances, provisionally called nebulium, 

 and are never known to show metals. The continuous spectra which 

 other nebulae present he refers to the solid or liquid portions set aglow 

 by collision. The luminosity of any given meteorite arising from 

 impact must be very transient, but the hypothesis assumes that the 

 aggregate result of many such collisions is a nearly constant emanation 



1 Astrophysical Journal, June, 1900, pp. 347, 348. 



